#include <iostream>
#include <queue>
using namespace std;

struct edge{
	int stop,next;
};

bool etf(int *e_header,edge *a,int start,int stop){
	int l;
	l = e_header[start];
	while(l != -1){
		if(a[l].stop == stop)
			return true;
		l = a[l].next;
	}
	return false;
}
//void log(int *d, int n){
//	for(int i=0;i<n;i++) cout<<d[i]<<" ";
//	cout<<endl;
//}
int main(){
	int n,m,s,t;
	int inf = 1e4;
	cin>>n>>m>>s>>t;
	s--;
	t--;
	edge edges[2*m];
	bool tf_s[n],tf_t[n];
	queue<int> q;
	int e_header[n];
	int fast_s[n],fast_t[n];
	for(int i=0;i<n;i++){
		e_header[i] = -1;
		tf_s[i] = true;
		tf_t[i] = true;
		fast_s[i] = inf;
		fast_t[i] = inf;
	}
	for(int i=0;i<m;i++){
		int u,v;
		cin>>u>>v;
		u--;
		v--;
		edges[i].stop = v;
		edges[i].next = e_header[u];
		e_header[u] = i;
		edges[i+m].stop = u;
		edges[i+m].next = e_header[v];
		e_header[v] = i+m;
	}
//从起点s开始的单元点最短路径 
	tf_s[s] = false;
	q.push(s);
	fast_s[s] = 0;
	while(!q.empty()){
		int l,len,stop;
		l = q.front();
		len = fast_s[l];
		tf_s[l] = true;
		q.pop();
		stop = e_header[l];
		while(stop != -1){
			if(fast_s[edges[stop].stop] > len+1){
				fast_s[edges[stop].stop] = len+1;
				if(tf_s[edges[stop].stop]){
					q.push(edges[stop].stop);
					tf_s[edges[stop].stop] = false;
				}
			}
			stop = edges[stop].next;
		}
	}
//从终点t开始的单元点最短路径 
	tf_t[t] = false;
	q.push(t);
	fast_t[t] = 0;
	while(!q.empty()){
		int l,len,stop;
		l = q.front();
		len = fast_t[l];
		tf_t[l] = true;
		q.pop();
		stop = e_header[l];
		while(stop != -1){
			if(fast_t[edges[stop].stop] > len+1){
				fast_t[edges[stop].stop] = len+1;
				if(tf_t[edges[stop].stop]){
					q.push(edges[stop].stop);
					tf_t[edges[stop].stop] = false;
				}
			}
			stop = edges[stop].next;
		}
	}
	int num=0;
	for(int i=0;i<n;i++)
		for(int j=0;j<i;j++)
			if(fast_s[i]+fast_t[j]+1 >= fast_s[t]  &&  fast_s[j]+fast_t[i]+1 >= fast_s[t]  &&  !etf(e_header,edges,i,j))
				num++;
	cout<<num;
//	cout<<endl<<"fast_s: "<<endl; log(fast_s, n);
//	cout<<"fast_t: "<<endl; log(fast_t, n);
	return 0;
}