// ~~ icebear ~~
#include <bits/stdc++.h>
using namespace std;
int n, m, s, t;
vector<int> G[200050];
int dist[200050];
 
int main() {
    ios_base::sync_with_stdio(0);
    cin.tie(0); cout.tie(0);
    cin >> n >> m >> s >> t;
    while(m--) {
        int u, v;
        cin >> u >> v;
        G[u].push_back(v);
        G[v].push_back(u);
    }
    for(int i = 1; i <= n; i++) dist[i] = 1000000000;
    dist[s] = 0;
    queue<int> q; // lưu xem là những đỉnh nào sẽ cần đi cập nhật
    q.push(s);
    while(!q.empty()) {
        int u = q.front(); q.pop(); // lấy ra đỉnh ở đầu hàng đợi (đây chính là đỉnh có khoảng cách tới s nhỏ nhất hiện tại)
        for(int v : G[u]) if (dist[v] > dist[u] + 1) {
            dist[v] = dist[u] + 1; // s->v khi đi qua đỉnh u và cạnh (u, v) sẽ tối ưu hơn
            q.push(v);
        }
    }
    if (dist[t] == 1000000000) // t chưa được thăm
        cout << -1;
    else
        cout << dist[t];
    return 0;
}
 

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