#include<cstdio>
#include<iostream>
#include<fstream>
using namespace std;
double sum,a[30];
int n,k,nr,st[30];
void back(int p)
{
    int ok,j,i;
    double P;
    for(i=1;i<=n;i++)
    {
        st[p]=i; ok=1;
        for(j=1;j<p;j++) if(st[j]==st[p]) {ok=0; j=p;}
        if(ok)
        {
            if(p==k)
            {
                P=1;
                for(j=1;j<=p;j++) P=P*a[st[j]];
                sum+=P;
                nr++;
            }
            else back(p+1);
        }
    }
}
int main()
{
    int i;
    freopen("dezastru.in","r",stdin);
    freopen("dezastru.out","w",stdout);
    scanf("%d %d",&n,&k);
    for(i=1;i<=n;i++) cin>>a[i];
    back(1);
    printf("%.6f",(double)(sum/nr));
}