#include <queue>

#include <cstdio>

#include <vector>

#include <cstring>

#include <algorithm>

#define N 1004

#define LL long long

#define pb push_back

#define setIO(s) freopen(s".in","r",stdin)

using namespace std;

int n,m;

queue<int>q;

vector<int>G[N];

double dp[N][N];

int nxt[N][N],dis[N][N],vis[N],d[N],vised[N][N];

double f(int a,int b)

{

    if(a==b)   return 0.0;

    if(nxt[a][b]==b)  return 1.00;

    if(nxt[nxt[a][b]][b]==b)   return 1.00;

    if(vised[a][b])   return dp[a][b];

    vised[a][b]=1;

    dp[a][b]=0.000;

    for(int i=0;i<G[b].size();++i)

    {

        int v=G[b][i];

        dp[a][b]+=(f(nxt[nxt[a][b]][b], v)+1.00)/(double)G[b].size();

    }

    return dp[a][b];

}

int main()

{

    int i,j,A,B;

    scanf("%d%d%d%d",&n,&m,&A,&B);

    for(i=1;i<=m;++i)

    {

        int a,b;

        scanf("%d%d",&a,&b);

        G[a].pb(b),G[b].pb(a);

    }

    for(i=1;i<=n;++i)

    {

        memset(vis,0,sizeof(vis));

        d[i]=0;

        q.push(i);

        vis[i]=1;

        while(!q.empty())

        {

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

            for(j=0;j<G[u].size();++j)

            {

                int v=G[u][j];

                if(!vis[v])

                {

                    vis[v]=1;

                    d[v]=d[u]+1;

                    q.push(v);

                }

            }

        }

        nxt[i][i]=i;

        for(j=1;j<=n;++j)

        {

            if(j==i)    continue;

            int mx=0,k,v;

            for(k=0;k<G[j].size();++k)

            {

                v=G[j][k];

                if(d[j]==d[v]+1&&(v<mx||!mx))     mx=v;

            }

            nxt[j][i]=mx;

        }

    }

    for(i=1;i<=n;++i)  G[i].pb(i);

    printf("%.3f\n",f(A,B));

    return 0;

}