#include <bits/stdc++.h>

using namespace std;

const int INF = 1<<30;

int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);

  int n, m, a, b;
  cin >> n >> m >> a >> b;
  --a, --b;

  vector<vector<int> > g(n + 1);
  for (int i = 0; i < m; i++) {
    int l, r;
    cin >> l >> r;
    --l, r;
    l = min(max(l, a), b + 1);
    r = min(max(r, a), b + 1);
    if (l != r) {
      g[l].push_back(r);
      g[r].push_back(l);
    }
  }

  vector<int> dist(n + 1, INF);
  dist[a] = 0;
  queue<int> q;
  q.push(a);

  while (!q.empty()) {
    int v = q.front();
    q.pop();
    for (int w: g[v]) {
      if (dist[w] == INF) {
        dist[w] = dist[v] + 1;
        q.push(w);
      }
    }
  }

  if (dist[b + 1] == INF)
    cout << -1 << endl;
  else
    cout << dist[b + 1] << endl;
  
  return 0;
}