#include<iostream>
#include<cstdio>
#include<cstring>
#include<algorithm>
#include<cmath>
#define File(s) freopen(s".in","r",stdin),freopen(s".out","w",stdout)
#define N 1010

using namespace std;
typedef double llg;

int n,E,C,M,dis[N],p[N][N],fr[N],d[N],ld;
int head[N],next[N<<1],to[N<<1],tt,du[N];
llg f[N][N]; bool vis[N],w[N][N];

void solve(int S){
	for(int i=1;i<=n;i++) dis[i]=vis[i]=0;
	vis[S]=1; d[ld=1]=S; du[S]++;
	for(int l=1,u;u=d[l],l<=ld;l++)
		for(int i=head[u],v;v=to[i],i;i=next[i])
			if(!vis[v]) dis[v]=dis[u]+1,vis[v]=1,d[++ld]=v,fr[v]=u;
			else if(dis[v]==dis[u]+1) fr[v]=min(fr[v],u);
	for(int i=1;i<=n;i++) p[S][i]=fr[i];
}

llg search(int u,int v){
	if(w[u][v]) return f[u][v];
	if(p[u][v]==u || p[u][p[u][v]]==u) return 1;
	w[u][v]=1;
	for(int i=head[u];i;i=next[i])
		f[u][v]+=search(to[i],p[u][p[u][v]])+1;
	f[u][v]+=search(u,p[u][p[u][v]])+1;
	return f[u][v]/=(llg)du[u];
}

int main(){
	File("a");
	scanf("%d %d %d %d",&n,&E,&C,&M);
	for(int i=1,x,y;i<=E;i++){
		scanf("%d %d",&x,&y); du[x]++; du[y]++;
		to[++tt]=y;next[tt]=head[x];head[x]=tt;
		to[++tt]=x;next[tt]=head[y];head[y]=tt;
	}
	for(int i=1;i<=n;i++) solve(i),w[i][i]=1;
	printf("%.3lf",search(M,C));
	return 0;
}