#include <iostream>
#include <algorithm>
#include <vector>
#include <queue>
 
using namespace std;
 
#define DEBUG(x) // cerr << #x << " = " << x << endl
 
const int INF = (int)1e9;
 
int main() {
	int N, M, S, T; cin >> N >> M >> S >> T;
	S--, T--;
	vector<vector<int> > E(N);
	for(int i = 0; i < M; i++) {
		int a, b; cin >> a >> b;
		a--, b--;
		E[a].push_back(b);
		E[b].push_back(a);
	}
 
	queue<int> Q;
 
	vector<int> distS(N, INF);
	vector<int> distSumS(N, 0);
 
	Q.push(S);
	distS[S] = 0;
	distSumS[0]++;
	while(!Q.empty()) {
		int cur = Q.front(); Q.pop();
		for(int i = 0; i < (int)E[cur].size(); i++) {
			int to = E[cur][i];
			if(distS[cur] + 1 < distS[to]) {
				Q.push(to);
				distS[to] = distS[cur] + 1;
				distSumS[distS[to]]++;
			}
		}
	}
 
	for(int i = 0; i < N; i++) {
		// cerr << "distS[" << i << "]=" << distS[i] << endl;
		// cerr << "distSumS[" << i << "]=" << distSumS[i] << endl;
	}
 
	Q = queue<int>();
	vector<int> distT(N, INF);
	vector<int> distSumT(N, 0);
 
	Q.push(T);
	distT[T] = 0;
	distSumT[0]++;
	while(!Q.empty()) {
		int cur = Q.front(); Q.pop();
		for(int i = 0; i < (int)E[cur].size(); i++) {
			int to = E[cur][i];
			if(distT[cur] + 1 < distT[to]) {
				Q.push(to);
				distT[to] = distT[cur] + 1;
				distSumT[distT[to]]++;
			}
		}
	}
 
	for(int i = 0; i < N; i++) {
		// cerr << "distT[" << i << "]=" << distT[i] << endl;
		// cerr << "distSumT[" << i << "]=" << distSumT[i] << endl;
	}
 
	int minDist = distS[T];
	DEBUG(minDist);
	long long ans = 0;
	for(int i = 0; i <= minDist - 2; i++) {
		ans += (long long)distSumS[i] * (long long)distSumT[minDist - i - 2];
	}
	cout << ans << endl;
}