#include <iostream>
#include <vector>
#include <queue>
using namespace std;
typedef vector<int> vi;
typedef pair<int,int> pii;
int INF = 1000000000;
vector<int> dijkstra(int s, int v, vector< vector< pair<int,int> > > G){
  //s: ?§????, v: ????????°, G: ??£??\??????????????????????????£??§??\????????????
  priority_queue< pair<int, int> > wait;
  vector<int> result(v, INF);
  result[s] = 0;
  wait.push(make_pair(0, s));
  while(!wait.empty()){
    int nowpoint = wait.top().second;
    int nowcost = wait.top().first;
    wait.pop();
    if(nowcost <= result[nowpoint]){
      for(int i = 0; i < G[nowpoint].size(); ++i){
	int nextpoint = G[nowpoint][i].second;
	int nextcost = -nowcost + G[nowpoint][i].first;
	if(nextcost < result[nextpoint]){
	  wait.push(make_pair(-nextcost, nextpoint));
	  result[nextpoint] = nextcost;
	}
      }
    }
  }
  return result;
}

long long int count(vector< vector<pii> > &G, int s, int t, int N){
  vector<int> r1 = dijkstra(s,N,G), r2 = dijkstra(t,N,G);
  int l = r1[t], d1[l+1], d2[l+1];
  fill(d1,d1+l+1,0);
  fill(d2,d2+l+1,0);
  for(int i = 0; i < N; ++i){
    if(r1[i] <= l) ++d1[r1[i]];
    if(r2[i] <= l) ++d2[r2[i]];
  }
  long long int ret = 0;
  for(int i = 0; i < l-1; ++i){
    ret += (long long int)d1[i]*d2[l-2-i];
  }
  return ret;
}

int main(){
  int N, M, s, t, x, y;
  cin >> N >> M >> s >> t;
  --s;--t;
  vector< vector<pii> > G(N);
  for(int i = 0; i < M; ++i){
    cin >> x >> y;
    --x;--y;
    G[x].push_back(make_pair(1,y));
    G[y].push_back(make_pair(1,x));
  }
  cout << count(G,s,t,N) << endl;
  return 0;
}