#include<iostream>

#include<cstdio>

#include<cstring>

#include<queue>

using namespace std;

int const maxn=;

int n,m,cc,kk,head[maxn],ct,dis[maxn][maxn],goal[maxn][maxn],inf;

double d[maxn],f[maxn][maxn],ans;

queue<int>q;

struct N{

    int to,next;

    N(int t=,int n=):to(t),next(n) {}

}edge[maxn<<];

void add(int x,int y){edge[++ct]=N(y,head[x]); head[x]=ct;}

void bfs(int s)

{

    while(q.size())q.pop();

    dis[s][s]=; q.push(s);

    while(q.size())

    {

        int x=q.front(); q.pop();

        for(int i=head[x];i;i=edge[i].next)

        {

            int u=edge[i].to;

            if(dis[s][u]==inf)

            {

                dis[s][u]=dis[s][x]+;

                q.push(u);

            }

        }

    }

}

double dfs(int c,int k)

{

    if(f[c][k]>=)return f[c][k];

    if(c==k)return f[c][k]=;

    if(dis[c][k]<=)return f[c][k]=;

    double P=1.0/(d[k]+1.0);

    f[c][k]=;//

    int to=goal[c][k];

    for(int i=head[k];i;i=edge[i].next)

        f[c][k]+=P*dfs(to,edge[i].to);

    f[c][k]+=P*dfs(to,k);

    return f[c][k];

}

int main()

{

    scanf("%d%d%d%d",&n,&m,&cc,&kk);

    for(int i=,x,y;i<=m;i++)

    {

        scanf("%d%d",&x,&y);

        add(x,y); add(y,x);

        d[x]+=1.0; d[y]+=1.0;

    }

    memset(dis,0x3f,sizeof dis); inf=dis[][];

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

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

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

        {

            if(dis[i][j]==inf)continue;

            if(dis[i][j]<=){goal[i][j]=j; continue;}

            int mx=inf,t=inf;

            for(int k=head[i];k;k=edge[k].next)

            {

                int u=edge[k].to;

                if(dis[u][j]<mx || (dis[u][j]==mx&&u<t))mx=dis[u][j],t=u;

            }

            int mx2=inf,t2=inf;

            for(int k=head[t];k;k=edge[k].next)

            {

                int u=edge[k].to;

                if(dis[u][j]<mx2 || (dis[u][j]==mx2&&u<t2))mx2=dis[u][j],t2=u;

            }

            goal[i][j]=t2;

        }

    memset(f,-,sizeof f);

    ans=dfs(cc,kk);

    printf("%.3lf",ans);

    return ;

}