#include <iostream>
#include <algorithm>
#include <vector>
#include <string>
#include <queue>

using namespace std;

using lint = long long;
using ldouble = long double;

const int INF = 1 << 30;

vector<int> bfs(const vector<vector<int>>& graph, int s, int t) {
    vector<int> dist(graph.size(), INF);
    dist[s] = 0;

    queue<int> que;
    que.push(s);
    while (!que.empty()) {
        int v = que.front();
        que.pop();

        for (int sv : graph[v]) {
            if (dist[sv] <= dist[v] + 1) continue;
            dist[sv] = dist[v] + 1;
            que.push(sv);
        }
    }
    return dist;
}

int main() {
    int N, M, s, t;
    cin >> N >> M >> s >> t;
    --s, --t;

    vector<vector<int>> graph(N);
    for (int i = 0; i < M; ++i) {
        int u, v;
        cin >> u >> v;
        --u, --v;

        graph[u].push_back(v);
        graph[v].push_back(u);
    }

    auto dist = bfs(graph, s, t);
    int D = dist[t];

    vector<lint> num(N, 0);
    for (int v = 0; v < N; ++v) {
        if (dist[v] < INF) ++num[dist[v]];
    }

    dist = bfs(graph, t, s);
    vector<lint> rnum(N, 0);
    for (int v = 0; v < N; ++v) {
        if (dist[v] < INF) ++rnum[dist[v]];
    }

    lint ans = 0;
    for (int d = 0; d <= D - 2; ++d) {
        ans += num[d] * rnum[D - d - 2];
    }

    cout << ans << endl;
    return 0;
}

