#include <bits/stdc++.h>
using namespace std;
int main(){
  int N, M, A, B;
  cin >> N >> M >> A >> B;
  vector<int> L(M), R(M);
  for (int i = 0; i < M; i++){
    cin >> L[i] >> R[i];
    L[i]--;
  }
  vector<vector<int>> E(N + 1);
  for (int i = 0; i < M; i++){
    E[L[i]].push_back(R[i]);
    E[R[i]].push_back(L[i]);
  }
  vector<int> d(N + 1, -1);
  queue<int> Q;
  for (int i = 0; i < A; i++){
    d[i] = 0;
    Q.push(i);
  }
  int ans = -1;
  while (!Q.empty()){
    int v = Q.front();
    Q.pop();
    if (v >= B){
      ans = d[v];
      break;
    }
    for (int w : E[v]){
      if (d[w] == -1){
        d[w] = d[v] + 1;
        Q.push(w);
      }
    }
  }
  cout << ans << endl;
}