#include<iostream>

#include<cstdio>

#include<cstring>

#include<queue>

#include<algorithm>

#define maxv 1050

#define maxe 2050

#define inf 1000000007

#define eps 1e-6

using namespace std;

int n,m,b,t,x,y,g[maxv],nume=;

int d[maxv],p[maxv][maxv],dis[maxv];

double f[maxv][maxv];

bool vis[maxv];

queue <int> q;

struct edge

{

    int v,nxt;

}e[maxe];

void addedge(int u,int v)

{

    e[++nume].v=v;

    e[nume].nxt=g[u];

    g[u]=nume;

}

void reset()

{

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

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

            p[i][j]=inf;

}

void pre_bfs(int x)

{

    while (!q.empty()) q.pop();

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

    {

        vis[i]=false;

        dis[i]=inf;

    }

    dis[x]=;

    for (int i=g[x];i;i=e[i].nxt)

    {

        int v=e[i].v;

        vis[v]=true;dis[v]=;q.push(v);

        p[x][v]=v;

    }

    while (!q.empty())

    {

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

        for (int i=g[head];i;i=e[i].nxt)

        {

            int v=e[i].v;

            if ((dis[v]>dis[head]+) || ((dis[v]==dis[head]+) && (p[x][v]>p[x][head])))

            {

                dis[v]=dis[head]+;

                p[x][v]=p[x][head];

                if (!vis[v]) q.push(v);

            }

        }

        vis[head]=false;

    }

    return;

}

double dp(int x,int y)

{

    if (x==y) return ;

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

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

    double ret=;

    for (int i=g[y];i;i=e[i].nxt)

    {

        int v=e[i].v;

        if (p[p[x][y]][y]==y) ret++;

        else if (p[x][y]==y) ret++;

        else ret+=dp(p[p[x][y]][y],v);

    }

    if (p[p[x][y]][y]==y) ret++;

    else if (p[x][y]==y) ret++;

    else ret+=dp(p[p[x][y]][y],y);

    ret/=(d[y]+);ret=ret+;

    f[x][y]=ret;

    return ret;

}

int main()

{

    memset(d,,sizeof(d));

    scanf("%d%d",&n,&m);scanf("%d%d",&b,&t);

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

    {

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

        addedge(x,y);addedge(y,x);

        d[x]++;d[y]++;

    }

    reset();

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

        pre_bfs(i);

    for (int i=;i<=n;i++) p[i][i]=inf;

    printf("%.3lf\n",dp(b,t));

    return ;

}