#include<bits/stdc++.h>

#define LL long long

#define LD long double

#define  N 100005

using namespace std;

inline int ra()

{

    int x=,f=; char ch=getchar();

    while (ch<'' || ch>'') {if (ch=='-') f=-; ch=getchar();}

    while (ch>='' && ch<='') {x=x*+ch-''; ch=getchar();}

    return x*f;

}

LD C[];

LD dp[][][];

int a[];

int main()

{

    int n=ra();

    C[]=;

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

    {

        C[i]=C[i-]*(LD)i;

        a[i]=ra();

    }

    sort(a+,a+n+);

    int m=ra();

    dp[][][]=; dp[][][a[]]=;

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

        for (int j=; j<=i; j++)

            for (int k=; k<=m; k++)

            {

                dp[i+][j+][k+a[i+]]+=dp[i][j][k];

                dp[i+][j][k]+=dp[i][j][k];

            }

    LD res=;

    for (int j=; j<=n; j++)

        for (int k=; k<=m; k++)

        {

            res+=dp[n][j][k]*(C[j]*C[n-j]);

            cout<<j<<"  "<<k<<"   "<<dp[n][j][k]<<endl;

        }

    res=res/C[n];

    cout<<res;

    return ;

}