func_code_string: #include <bits/stdc++.h>   using namespace std;   vector<int> edge[1005];   int vis[2][1005];   int d[2][1005];   int adj[1005][1005];   void djikstra(int s, int id) {     priority_queue<pair<int, int> > PQ;     PQ.push(make_pair(0, s));     d[id][s] = 0;     while (!PQ.empty()) {       pair<int, int> p = PQ.top();       PQ.pop();       int v = p.second;       int cd = -p.first;       if (cd != d[id][v] || vis[id][v]) continue;       vis[id][v] = 1;       for (int i = 0; i < edge[v].size(); i++) {         int u = edge[v][i];         int nd = cd + 1;         if (!vis[id][u] && nd < d[id][u]) {           d[id][u] = nd;           PQ.push(make_pair(-nd, u));         }       }     }   }   int main() {     int n, m, s, t;     scanf( %d%d%d%d , &n, &m, &s, &t);     for (int i = 0; i < m; i++) {       int u, v;       scanf( %d%d , &u, &v);       edge[u].push_back(v);       edge[v].push_back(u);       adj[u][v] = adj[v][u] = 1;     }     for (int i = 1; i <= n; i++) {       d[0][i] = d[1][i] = n;     }     djikstra(s, 0);     djikstra(t, 1);     int th = d[0][t];     int ans = 0;     for (int i = 1; i <= n; i++) {       for (int j = i + 1; j <= n; j++) {         if (adj[i][j]) continue;         int dist = min(d[0][i] + d[1][j], d[0][j] + d[1][i]);         if (dist + 1 >= th) ans++;       }     }     printf( %d n , ans);   }   