/**************************************************************

     Problem: 1415

     User: Tunix

     Language: C++

     Result: Accepted

     Time:208 ms

     Memory:17240 kb

 ****************************************************************/

 //BZOJ 1415

 #include<cstdio>

 #include<queue>

 #include<cstring>

 #include<cstdlib>

 #include<iostream>

 #include<algorithm>

 #define rep(i,n) for(int i=0;i<n;++i)

 #define F(i,j,n) for(int i=j;i<=n;++i)

 #define D(i,j,n) for(int i=j;i>=n;--i)

 #define pb push_back

 using namespace std;

 typedef long long LL;

 inline int getint(){

     int r=,v=; char ch=getchar();

     for(;!isdigit(ch);ch=getchar()) if (ch=='-') r=-;

     for(; isdigit(ch);ch=getchar()) v=v*-''+ch;

     return r*v;

 }

 const int N=,M=;

 /*******************template********************/

 struct edge{int to,next;}E[M<<];

 int head[N],cnt;

 void add(int x,int y){

     E[++cnt]=(edge){y,head[x]}; head[x]=cnt;

     E[++cnt]=(edge){x,head[y]}; head[y]=cnt;

 }

 int n,m,S,T,p[N][N],d[N][N],du[N];

 queue<int>Q;

 void bfs(int x){

     Q.push(x);

     d[x][x]=;

     while(!Q.empty()){

         int now=Q.front(),tmp=p[x][now]; Q.pop();

         for(int i=head[now];i;i=E[i].next)

             if (d[x][E[i].to]==- ||

                     (d[x][now]+==d[x][E[i].to] && tmp<p[x][E[i].to])){

                 d[x][E[i].to]=d[x][now]+;

                 p[x][E[i].to]=tmp;

                 if (!tmp) p[x][E[i].to]=E[i].to;

                 Q.push(E[i].to);

             }

     }

 }

 double f[N][N];

 double dp(int x,int y){

     if (f[x][y]) return f[x][y];

     if (x==y) return ;

     if (p[x][y]==y || p[p[x][y]][y]==y) return f[x][y]=;

     double tot=dp(p[p[x][y]][y],y);

     for(int i=head[y];i;i=E[i].next)

         tot+=dp(p[p[x][y]][y],E[i].to);

     return f[x][y]=tot/(du[y]+)+;

 }

 int main(){

 #ifndef ONLINE_JUDGE

     freopen("1415.in","r",stdin);

     freopen("1415.out","w",stdout);

 #endif

     memset(d,-,sizeof d);

     n=getint(); m=getint();

     S=getint(); T=getint();

     F(i,,m){

         int x=getint(),y=getint();

         add(x,y);

         du[x]++; du[y]++;

     }

     F(i,,n) bfs(i);

     printf("%.3f\n",dp(S,T));

     return ;

 }