//  Copyright © 2017 Adán López Alatorre. All rights reserved.
//

#include <iostream>
#include <vector>
#include <string.h>
#include <deque>
#include <queue>
#include <algorithm>
#include <set>
#include <map>
#include <sstream>
#include <stack>
#include <iomanip>
#include <fstream>
#include <climits>
#include <cmath>
#define fi first
#define se second
#define FI first.first
#define SE first.second
#define TH second
#define MAXN maxN

using namespace std;

typedef long long ll;
typedef pair<int, int> ii;
typedef pair<ii, int> iii;
typedef pair<double, int> di;
typedef vector<int> vi;
typedef set<ii>::iterator iter;

const int maxN = 2e3 + 3, MOD = 1e9 + 7, AL = 10;

int toh[maxN], frh[maxN];
bool mat[maxN][maxN];

vector<int> adj[maxN];

void bfs(int s, int *arr){
    queue<ii> q;
    q.push(ii(s, 0));

    memset(arr, -1, sizeof(toh));
    arr[s] = 0;

    while(q.size()){
        ii cur = q.front(); q.pop();

        for(int sn: adj[cur.fi])
            if(arr[sn] == -1)
                arr[sn] = cur.se + 1, q.push(ii(sn, cur.se + 1));
    }

}

int main(){
    ios::sync_with_stdio(false);
    cin.tie(0), cout.tie(0);

    int n, m, s, t;
    cin >> n >> m >> s >> t;
    s--, t--;

    for(int i = 0, u, v; i < m; i++){
        cin >> u >> v;
        u--, v--;
        adj[u].push_back(v), adj[v].push_back(u);
        mat[u][v] = mat[v][u] = true;
    }

    bfs(s, toh), bfs(t, frh);

    int res = 0;

    for(int i = 0; i < n; i++){
        for(int j = i + 1; j < n; j++){
            if(mat[i][j]) continue;
            int bes = min(toh[i] + frh[j] + 1, toh[j] + frh[i] + 1);
            if(bes >= toh[t]) res++;
        }
    }
    cout << res << '\n';

}