#include<iostream>

 #include<stdio.h>

 #include<algorithm>

 #include<iomanip>

 #include<cmath>

 #include<cstring>

 #include<vector>

 #define ll __int64

 #define pi acos(-1.0)

 #define MAX 50000

 using namespace std;

 int c[],n,ff;

 double dp[];

 double solve(int f)

 {

     if(dp[f]>) return dp[f];

     dp[f]=;

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

         if(f>c[i]){

             double t=(sqrt(5.0)+1.0)/*c[i]*c[i];

             int p=(int)t;

             dp[f]+=1.0*p/n;

         }

         else{

             dp[f]+=(+solve(f+c[i]))/n;

         }

     }

     return dp[f];

 }

 int main(){

     while(scanf("%d%d",&n,&ff)!=EOF){

         for(int i=;i<n;i++) cin>>c[i];

         memset(dp,,sizeof(dp));

         printf("%.3lf\n",solve(ff));

     }

     return ;

 }