#include <bits/stdc++.h>
using namespace std;
#define IO ios_base::sync_with_stdio(0), cin.tie(0), cout.tie(0);
const long long N = 2e3 + 5, MOD = 1e9 + 7, OO = 1e18;
const double PI = acos(-1);
const int dx[4] = {0, 0, 1, -1};
const int dy[4] = {1, -1, 0, 0};

long long n, m, s, t, u, v;
vector<long long>g[N];
pair<long long, long long>sd[N];
bool vis[N][N];

pair<long long, long long> bfs(long long x) {
    bool used[n + 1] = {};
    pair<long long, long long>ret = {0, 0};
    queue<long long>q;
    q.push(x);
    used[x] = 1;
    int l = 1, rep;
    while (rep = q.size()) {
        while (rep--) {
            long long y = q.front();
            q.pop();
            for (auto z : g[y])
                if (!used[z]) {
                    q.push(z);
                    used[z] = 1;
                    if (z == s)ret.first = l;
                    else if (z == t)ret.second = l;
                    if (used[s] && used[t])return ret;
                }
        }
        l++;
    }
    return {0, 0};
}

int main() {
    IO
    cin >> n >> m >> s >> t;
    for (int i = 1; i <= m; i++) {
        cin >> u >> v;
        g[u].push_back(v);
        g[v].push_back(u);
        vis[u][v] = vis[v][u] = 1;
    }
    long long ans = 0;
    for (int i = 1; i <= n; i++) {
        for (int j = 1; j <= n; j++) {
            if (i == j || vis[i][j])continue;
            vis[i][j] = vis[j][i] = 1;
            long long c1 = sd[i].first + sd[j].second + 1;
            long long c2 = sd[i].second + sd[j].first + 1;
            ans += (min(c1, c2) <= sd[s].second);
        }
    }
    cout << ans;
    return 0;
}