#include <bits/stdc++.h>
typedef long long ll;
using namespace std;
#define forn(i,n) for(int i=0;i!=n;i++)
ll n,c,g,a,ac,L[7],m,k,dp[500][500];
int main(){
cin>>n>>c,c*=n,m=n-1,k=1<<m;
forn(i,n)cin>>L[i],L[i]*=n;
sort(L,L+n);
do{
    memset(dp,0,sizeof(dp)),dp[L[m]][0]=1;
    forn(i,c)if(i%n){
        forn(mask,k)if(!((1<<(i%n-1))&mask))
            for(ac=i;ac<=c;ac++)dp[max(ac,min(c,L[i%n-1]+i))][mask+(1<<(i%n-1))]+=dp[ac][mask];
    }g+=dp[c][k-1],a++;
}while(next_permutation(L,L+m));
printf("%.15f",g/pow(c/n,m)/a);
}
