import java.io.*;
import java.util.*;

public class Main {

    public static void main(String[] args) {
        InputReader in = new InputReader(System.in);
        OutputWriter out = new OutputWriter(System.out);
        TaskC solver = new TaskC(in, out);
        solver.solve();
        out.close();
    }

    static class TaskC {

        static int n;
        static int m;

        static int[] inv;
        static int[] fact;
        static int[] invfact;
        
        static int[] powN;
        static int[] powM;

        InputReader in;
        OutputWriter out;

        TaskC(InputReader in, OutputWriter out) {
            this.in = in;
            this.out = out;
        }
        
        private void solve() {
            n = in.readInt();
            m = in.readInt();
            precalc();
            int ans = 0;
            int choosePath = 1;
            for(int k = 1; k < n; ++k) {
                if (k > m) break;
                int cur = MathUtil.mul(choosePath, cayley(n, k + 1));
                cur = MathUtil.mul(cur, binom(m - 1, k - 1));
                cur = MathUtil.mul(cur, powM[n - k - 1]);
                choosePath = MathUtil.mul(choosePath, n - k - 1);
                ans = MathUtil.sum(ans, cur);
            }
            out.print(ans);
        }

        private int cayley(int n, int k) {
            if (n - k - 1 < 0) {
                return MathUtil.mul(k, MathUtil.binPow(n, MathUtil.mod - 2));
            }
            return MathUtil.mul(k, powN[n - k - 1]);
        }

        private int binom(int n, int k) {
            return MathUtil.mul(fact[n], MathUtil.mul(invfact[k], invfact[n - k]));
        }

        private void precalc() {
            powN = new int[n + 1];
            powM = new int[n + 1];
            powN[0] = 1;
            powM[0] = 1;
            for(int i = 1; i <= n; ++i) {
                powN[i] = MathUtil.mul(powN[i - 1], n);
                powM[i] = MathUtil.mul(powM[i - 1], m);
            }
            
            inv = new int[m + 1];
            fact = new int[m + 1];
            invfact = new int[m + 1];
            inv[0] = inv[1] = 1;
            fact[0] = fact[1] = 1;
            invfact[0] = invfact[1] = 1;
            for(int i = 2; i <= m; ++i) {
                inv[i] = MathUtil.mul(inv[MathUtil.mod % i], MathUtil.mod - MathUtil.mod / i);
                fact[i] = MathUtil.mul(fact[i - 1], i);
                invfact[i] = MathUtil.mul(invfact[i - 1], inv[i]);
            }
        }
    }
    static class MathUtil {
        private static final int mod = 1_000_000_007;

        static int binPow(int a, int n) {
            int result = 1;
            while (n > 0) {
                if (n % 2 == 1) {
                    result = mul(result, a);
                }
                n /= 2;
                a = mul(a, a);
            }
            return result;
        }
        
        static int mul(int a, int b) {
            return (int)((long)a * b % mod);
        }

        static int sum(int a, int b) {
            int result = a + b;
            if (result >= mod) result -= mod;
            return result;
        }

    }

    static class OutputWriter {
        private final PrintWriter writer;

        public OutputWriter(OutputStream outputStream) {
            writer = new PrintWriter(new BufferedWriter(new OutputStreamWriter(outputStream)));
        }

        public OutputWriter(Writer writer) {
            this.writer = new PrintWriter(writer);
        }

        public void print(Object... objects) {
            for (int i = 0; i < objects.length; i++) {
                if (i != 0) {
                    writer.print(' ');
                }
                writer.print(objects[i]);
            }
            writer.println();
        }

        public void close() {
            writer.close();
        }

    }

    static class InputReader {
        private InputStream stream;
        private byte[] buf = new byte[1024];
        private int curChar;
        private int numChars;
        private InputReader.SpaceCharFilter filter;

        public InputReader(InputStream stream) {
            this.stream = stream;
        }

        public int read() {
            if (numChars == -1) {
                throw new InputMismatchException();
            }
            if (curChar >= numChars) {
                curChar = 0;
                try {
                    numChars = stream.read(buf);
                } catch (IOException e) {
                    throw new InputMismatchException();
                }
                if (numChars <= 0) {
                    return -1;
                }
            }
            return buf[curChar++];
        }

        public int readInt() {
            int c = read();
            while (isSpaceChar(c)) {
                c = read();
            }
            int sgn = 1;
            if (c == '-') {
                sgn = -1;
                c = read();
            }
            int res = 0;
            do {
                if (c < '0' || c > '9') {
                    throw new InputMismatchException();
                }
                res *= 10;
                res += c - '0';
                c = read();
            } while (!isSpaceChar(c));
            return res * sgn;
        }

        public boolean isSpaceChar(int c) {
            if (filter != null) {
                return filter.isSpaceChar(c);
            }
            return isWhitespace(c);
        }

        public static boolean isWhitespace(int c) {
            return c == ' ' || c == '\n' || c == '\r' || c == '\t' || c == -1;
        }

        public double readDouble() {
            int c = read();
            while (isSpaceChar(c)) {
                c = read();
            }
            int sgn = 1;
            if (c == '-') {
                sgn = -1;
                c = read();
            }
            double res = 0;
            while (!isSpaceChar(c) && c != '.') {
                if (c == 'e' || c == 'E') {
                    return res * Math.pow(10, readInt());
                }
                if (c < '0' || c > '9') {
                    throw new InputMismatchException();
                }
                res *= 10;
                res += c - '0';
                c = read();
            }
            if (c == '.') {
                c = read();
                double m = 1;
                while (!isSpaceChar(c)) {
                    if (c == 'e' || c == 'E') {
                        return res * Math.pow(10, readInt());
                    }
                    if (c < '0' || c > '9') {
                        throw new InputMismatchException();
                    }
                    m /= 10;
                    res += (c - '0') * m;
                    c = read();
                }
            }
            return res * sgn;
        }

        public interface SpaceCharFilter {
            public boolean isSpaceChar(int ch);
        }

    }
}