#include <iostream>

 #include <cstdio>

 #include <algorithm>

 #include <list>

 #include <map>

 #include <stack>

 #include <vector>

 #include <cstring>

 #include <sstream>

 #include <string>

 #include <cmath>

 #include <queue>

 using namespace std;

 #define clc(a,b) memset(a,b,sizeof(a))

 #define inf 0x3f3f3f3f

 const int N=;

 const int MOD = 1e9+;

 #define LL long long

 void fre() {

     freopen("in.txt","r",stdin);

 }

 inline int r() {

     int x=,f=;char ch=getchar();

     while(ch>''||ch<'') {if(ch=='-') f=-;ch=getchar();}

     while(ch>=''&&ch<='') { x=x*+ch-'';ch=getchar();}return x*f;

 }

 vector<int>g[];

 LL countt[];

 LL val[]={};

 int main(){

     int n,m,s,t;

     n=r();

     m=r();

     s=r();

     t=r();

     for(int i=;i<=n;i++)

         g[i].clear();

     for(int i=;i<m;i++){

         int x,y;

         x=r();

         y=r();

         g[x].push_back(y);

         g[y].push_back(x);

     }

     val[s]=;

     for(int i=;i<=t;i++){

         for(int j=;j<n;j++)

             countt[j]=;

         for(int j=;j<n;j++){

             LL num=val[j];

             for(int p=;p<(int)g[j].size();p++){

                 int v=g[j][p];

                 countt[v]+=num;

             }

         }

         for(int j=;j<n;j++){

             val[j]=countt[j];

         }

     }

     LL ans=;

     for(int i=;i<n;i++)

         ans+=val[i];

     printf("%lld\n",ans);

     return ;

 }