#include <cstdio>
#include <stack>
 
using namespace std;
 
int N, M;
int A, B;
int parent[101];
 
stack<int> SA;
stack<int> SB;
 
int main() {
    scanf("%d %d %d %d", &N, &A, &B, &M);
 
    for (int i = 0; i < M; i ++) {
        int x, y;
        scanf("%d %d", &x, &y);
        parent[y] = x;
    }
 
    SA.push(A);
    while (parent[SA.top()] != 0) {
        SA.push(parent[SA.top()]);
    }
 
    SB.push(B);
    while (parent[SB.top()] != 0) {
        SB.push(parent[SB.top()]);
    }
 
    if (SA.top() != SB.top()) {
        printf("-1\n");
    } else {
        while (!SA.empty() && !SB.empty() && SA.top() == SB.top()) {
            SA.pop();
            SB.pop();
        }
        printf("%d\n", (int) SA.size() + (int) SB.size());
    }
 
    return 0;
}

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

OQo3IDMKNwoxIDIKMSAzCjIgNwoyIDgKMiA5CjQgNQo0IDYK

9
7 3
7
1 2
1 3
2 7
2 8
2 9
4 5
4 6