#include <bits/stdc++.h>

using namespace std;

using ll = long long;

constexpr char newl = '\n';

int main() {
    cin.tie(nullptr);
    ios::sync_with_stdio(false);

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

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

    queue<int> q;
    vector<int> d(n + 1, -1);
    for (int i = 0; i <= a; i++) {
        d[i] = 0;
        q.push(i);
    }

    while (!q.empty()) {
        int cur = q.front();
        q.pop();
        for (int nex : g[cur]) {
            if (d[nex] != -1) continue;
            d[nex] = d[cur] + 1;
            q.push(nex);
        }
    }

    int ans = 1e9;
    for (int i = b; i <= n; i++) {
        ans = min(ans, (d[i] == -1 ? 1000000000 : d[i]));
    }
    cout << (ans >= 1000000000 ? -1 : ans) << newl;
    return 0;
}