#include <iostream>
#include <cstdio>
#include <cstring>
#include <algorithm>
#include <cmath>
using namespace std;
typedef long long ll;
const int N=1005;
inline int read(){
    char c=getchar();int x=0,f=1;
    while(c<‘0‘||c>‘9‘){if(c==‘-‘)f=-1;c=getchar();}
    while(c>=‘0‘&&c<=‘9‘){x=x*10+c-‘0‘;c=getchar();}
    return x*f;
}

int n,m,a,b,de[N],u,v;
struct edge{
    int v,ne;
}e[N<<1];
int h[N],cnt;
inline void ins(int u,int v){
    cnt++;
    e[cnt].v=v;e[cnt].ne=h[u];h[u]=cnt;
    cnt++;
    e[cnt].v=u;e[cnt].ne=h[v];h[v]=cnt;
}

int g[N][N];
int q[N],head,tail,d[N][N];
bool inq[N];
inline void lop(int &x){if(x==N) x=1;}
void bfs(int s,int *g,int *d){
    head=tail=1;
    memset(inq,0,sizeof(inq));
    q[tail++]=s; d[s]=0; inq[s]=1;
    while(head!=tail){
        int u=q[head++];inq[u]=0;lop(head);
        for(int i=h[u];i;i=e[i].ne){
            int v=e[i].v;
            if(d[v]>d[u]+1 || (d[v]==d[u]+1&&g[v]>g[u])){
                d[v]=d[u]+1;
                g[v]= g[u]==0 ? v : g[u];
                if(!inq[v]) q[tail++]=v,inq[v]=1,lop(tail);
            }
        }
    }
}

double f[N][N];
double dfs(int a,int b){//printf("dfs %d %d\n",a,b);
    if(a==b) return 0;
    if(g[a][b]==b || g[ g[a][b] ][b]==b) return 1;
    if(f[a][b]) return f[a][b];
    double &re=f[a][b];
    int t=g[ g[a][b] ][b];
    re=dfs(t,b);
    for(int i=h[b];i;i=e[i].ne)    re+=dfs(t,e[i].v);
    re=re/(de[b]+1)+1;
    return re;
}

int main(){
    freopen("in","r",stdin);
    n=read();m=read();a=read();b=read();
    for(int i=1;i<=m;i++) u=read(),v=read(),de[u]++,de[v]++,ins(u,v);
    memset(d,127,sizeof(d));
    for(int i=1;i<=n;i++) bfs(i,g[i],d[i]);

    //for(int i=1;i<=n;i++) for(int j=1;j<=n;j++) printf("g %d %d  %d\n",i,j,g[i][j]);
    printf("%.3lf",dfs(a,b));
}