#include <bits/stdc++.h>

using namespace std;
ifstream fin("dezastru.in");
ofstream fout("dezastru.out");
int n,i,k,nr,v[26];
float pr,sum,a[26];
int main()
{
    fin>>n>>k;
    for(i=1; i<=n; i++)
    {
        fin>>a[i];
        v[i]=i;
    }

    do
    {pr=1;
        for(i=1; i<=k; i++)
            pr*=a[v[i]];
        sum+=pr;
        nr++;
    }
    while(next_permutation(v+1,v+n+1));
fout<<setprecision(6)<<sum/nr;
    return 0;
}