#include <iostream>

#include <cstdio>

#include <cmath>

#include <algorithm>

#include <cstring>

using namespace std;

double dp[20005];

int ci[125];

double is=0.5*(1+sqrt(5));

int main(){

	int n,f,ma;

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

		ma=-1;

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

			scanf("%d",&ci[i]);

			ma=max(ma,ci[i]);

		}

		ma=ma*2;

		for(int i=max(ma,f);i>=f;i--){

			dp[i]=0;

			for(int k=1;k<=n;k++){

				if(i>ci[k]) dp[i]+=(int)(is*ci[k]*ci[k]);

				else{

					dp[i]+=(dp[i+ci[k]]+1);

				}

			}

			dp[i]=(dp[i])/n;

		}

		printf("%.3lf\n",dp[f]);

	}

	return 0;

}