#include <bits/stdc++.h>

using namespace std;

int main() {
  ios::sync_with_stdio(false);
  cin.tie(nullptr);
  int n, k;
  string s, t;
  cin >> n >> k >> s >> t;
  const int md = 1e9 + 7;
  auto mul = [&](int a, int b) -> int {
    return (long long) a * b % md;
  };
  auto add = [&](int a, int b) {
    return (a += b - md) < 0 ? a + md : a;
  };
  auto inv = [&](int a) {
    int s = 1, t = 0, b = md;
    while (b) {
      int q = a / b;
      swap(s -= q * t, t);
      swap(a -= q * b, b);
    }
    return s < 0 ? s + md : s;
  };
  vector<int> pw(k + 1);
  pw[0] = 1;
  for (int i = 0; i < k; i++) {
    pw[i + 1] = mul(pw[i], 25);
  }
  vector<int> fact(n + 1), ifact(n + 1);
  fact[0] = 1;
  for (int i = 0; i < n; i++) {
    fact[i + 1] = mul(i + 1, fact[i]);
  }
  ifact[n] = inv(fact[n]);
  for (int i = n - 1; i >= 0; i--) {
    ifact[i] = mul(i + 1, ifact[i + 1]);
  }
  auto binom = [&](int n, int m) {
    if (n < m) {
      return 0;
    }
    return mul(fact[n], mul(ifact[n - m], ifact[m]));
  };
  int ans = 0;
  for (int i = 0; i < n; i++) {
    if (k == 0) {
      break;
    }
    int d;
    int ti = t[i] - 'a';
    int si = s[i] - 'a';
    if (ti <= si) {
      d = mul(ti, mul(pw[k - 1], binom(n - i - 1, k - 1)));
    } else {
      d = mul(pw[k], binom(n - i - 1, k));
      if (ti > 1) {
        d = add(d, mul(ti - 1, mul(binom(n - i - 1, k - 1), pw[k - 1])));
      }
    }
    ans = add(ans, d);
    k -= (si != ti);
  }
  cout << add(ans, 1) << endl;
  cerr << "Time: " << double(clock()) / CLOCKS_PER_SEC << '\n';
  return 0;
}