#include<bits/stdc++.h>
using namespace std;
using LL = long long;
using ULL = unsigned long long;
#define rep(i,n) for(int i=0; i<(n); i++)

int N, M;
int A, B;
vector<int> nx[100001];
int dp[100001];

const int INF = 100000000;

int main() {
    cin >> N >> M;
    cin >> A >> B; A--; B--;
    rep(i, M) {
        int l, r; cin >> l >> r; l--;
        nx[l].push_back(r);
        nx[r].push_back(l);
    }

    rep(i, N + 1) dp[i] = INF;
    rep(i, A + 1) dp[i] = 0;
    queue<int> Q; rep(i, A + 1) Q.push(i);
    while (Q.size()) {
        int p = Q.front(); Q.pop();
        for (int v : nx[p]) {
            if (dp[v] < INF) continue;
            dp[v] = dp[p] + 1;
            Q.push(v);
        }
    }

    int ans = INF;
    for (int i = B + 1; i <= N; i++) ans = min(ans, dp[i]);
    if (ans >= INF / 2) cout << -1 << endl;
    else cout << ans << endl;

    return 0;
}