#include <bits/stdc++.h>

using namespace std;
ifstream fin("dezastru.in");
ofstream fout("dezastru.out");
double sum,v[30];
int cnt,viz[30],n,i,k,s[30];
void bkt(int pas,double p)
{
    int i;
    for(i=s[pas-1]+1;i<=n;i++)
    {
        if(viz[i]==0)
        {
            viz[i]=1;
            s[pas]=i;
            if(pas==k) {cnt++;sum+=p*v[i];}
            else bkt(pas+1,1.0*p*v[i]);
            viz[i]=0;
        }
    }
}
int main()
{
    fin>>n>>k;
    for(i=1;i<=n;i++)
        fin>>v[i];
    bkt(1,1);

    fout<<(double)sum/cnt;
    return 0;
}