#include<queue>

#include<cstdio>

#include<algorithm>

using namespace std;

int read_p,read_ca;

inline int read(){

    read_p=;read_ca=getchar();

    while(read_ca<''||read_ca>'') read_ca=getchar();

    while(read_ca>=''&&read_ca<='') read_p=read_p*+read_ca-,read_ca=getchar();

    return read_p;

}

struct na{

    int y,ne;

}b[];

queue <int> q;

int a,bb;

int n,m,l[],r[],ru[],num=;

int ne[][],dis[][];

double jy[][];

bool bo[][];

const int INF=1e9;

inline void add(int x,int y){

    num++;

    if (!l[x]) l[x]=num;else b[r[x]].ne=num;

    b[num].y=y;r[x]=num;

}

inline void bfs(int x){

    for (int i=;i<=n;i++) dis[x][i]=INF;

    dis[x][x]=;ne[x][x]=x;

    for (int i=l[x];i;i=b[i].ne) dis[x][b[i].y]=,ne[x][b[i].y]=b[i].y,q.push(b[i].y);

    while (!q.empty()){

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

        for (int i=l[k];i;i=b[i].ne)

        if (dis[x][b[i].y]>dis[x][k]+) dis[x][b[i].y]=dis[x][k]+,ne[x][b[i].y]=ne[x][k],q.push(b[i].y);else

        if (dis[x][b[i].y]==dis[x][k]+&&ne[x][b[i].y]>ne[x][k]) ne[x][b[i].y]=ne[x][k],q.push(b[i].y);

    }

}

inline double dfs(int a,int bb){

    if (a==bb) return 0.0;

    if (bo[a][bb]) return jy[a][bb];

    bo[a][bb]=;

    double ans=0.0;

    int aa=ne[ne[a][bb]][bb];

    if (aa==bb) return jy[a][bb]=1.0;

    int u=;

    for (int i=l[bb];i;i=b[i].ne) ans+=dfs(aa,b[i].y),u++;

    ans+=dfs(aa,bb);

    ans=1.0/u*ans+1.0;

    return jy[a][bb]=ans;

}

int main(){

    int i,j,x,y;

    n=read();m=read();

    a=read();bb=read();

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

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

    printf("%.3lf\n",dfs(a,bb));

    return ;

}