#include<bits/stdc++.h>

using namespace std;

const int N = 1e3 + 5;

int n, m, s, f;
int used[N], is[N];
vector <int> g[N];

void dfs(int v, int pr) {
    if (v == f) return;
    used[v] = 1;
    for (int u : g[v]) {
        if (u == pr) continue;
        if (!used[u]) {
            dfs(u, v);
        }
        if (is[u]) {
            is[v] = 1;
        }
    }
}

int main(){
    cin >> n >> m >> s >> f;

    for (int i = 1; i <= m; i++) {
        int u, v;
        cin >> u >> v;
        g[u].push_back(v);
        g[v].push_back(u);
    }

    is[f] = 1;
    dfs(s, -1);

    int res = 0;
    for (int i = 1; i <= n; i++) {
        if (!is[i]) {
            res++;
        }
    }

    cout << res << endl;
}