#include <bits/stdc++.h>   using namespace std;   int n, m, s, t, d[200005], v[200005], dd[200005], ma[1005][1005], ok = 1, y,                                                                     ans;   int main() {     scanf( %d%d%d%d , &n, &m, &s, &t);     for (int i = 1; i <= m; i++) {       int x, y;       scanf( %d%d , &x, &y);       ma[x][y] = ma[y][x] = 1;     }     for (int i = 1; i <= n; i++) d[i] = 1000000000, v[i] = 0;     d[s] = 0;     for (int i = 1; i <= n; i++) {       int minv = 1000000000, u;       for (int j = 1; j <= n; j++)         if (!v[j] && d[j] < minv) {           minv = d[j];           u = j;         }       v[u] = 1;       for (int k = 1; k <= n; k++)         if (!v[k] && ma[u][k] && d[u] + ma[u][k] < d[k]) d[k] = d[u] + ma[u][k];     }     for (int i = 1; i <= n; i++) dd[i] = 1000000000, v[i] = 0;     dd[t] = 0;     for (int i = 1; i <= n; i++) {       int minv = 1000000000, u;       for (int j = 1; j <= n; j++)         if (!v[j] && dd[j] < minv) {           minv = dd[j];           u = j;         }       v[u] = 1;       for (int k = 1; k <= n; k++)         if (!v[k] && ma[u][k] && dd[u] + ma[u][k] < dd[k])           dd[k] = dd[u] + ma[u][k];     }     for (int i = 1; i < n; i++)       for (int j = i + 1; j <= n; j++)         if (!ma[i][j]) {           if (d[i] + dd[j] + 1 >= d[t] && d[j] + dd[i] + 1 >= d[t]) {             ans++;           }         }     printf( %d , ans);   }   