6

#include<bits/stdc++.h>

using namespace std;

const int N = 105;

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

void dfs(int v) {
    used[v] = 1;
    for (int u : g[v]) {
        if (!used[u])
            dfs(u);
    }
}

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);
    }

    dfs(s);

    vector <int> comp;

    for (int i = 1; i <= n; i++) {
        if (used[i]) comp.push_back(i);
    }

    int ans = n - comp.size();

    for (int c : comp) {
        memset(used, 0, sizeof(used));
        used[f] = 1;
        dfs(c);
        if (!used[s]) {
            ans++;
            continue;
        }
        memset(used, 0, sizeof(used));
        used[s] = 1;
        dfs(c);
        if (!used[f]) {
            ans++;
            continue;
        }
        for (int d = 1; d <= n; d++) {
            if (d == c || d == f || d == s) continue;
            int cnt_comp = 0;
            memset(used, 0, sizeof(used));
            used[d] = 1;
            dfs(c);
            if (!used[f] && !used[s]) cnt_comp++;
            memset(used, 0, sizeof(used));
            used[d] = 1;
            dfs(f);
            if (!used[c] && !used[s]) cnt_comp++;
            memset(used, 0, sizeof(used));
            used[d] = 1;
            dfs(s);
            if (!used[f] && !used[c]) cnt_comp++;
            if (cnt_comp == 3) {
                ans++;
                break;
            }
        }
    }

    cout << ans << endl;
}