#include <iostream>
#include <fstream>
#include <iomanip>
using namespace std;

ifstream fin("dezastru.in");
ofstream fout("dezastru.out");

int k,n,fact=1;
double v[26];
double sum=0.0, sol=1;

void bkt(int j, int cont){
    for (int i=j; i<=n; ++i){
        //cout<<i<<' '<<sol<<' ';
        sol*=v[i];
        //if (i==k-1) sum/=fact;
        if (cont<k)
            bkt(i+1,cont+1);
        else {
            //cout<<' '<<sol<<'\n';
            sum+=sol/fact;
            sol/=v[i];
        }
    }
    sol/=v[j-1];
    //cout<<sol<<' ';
}

int main(){
    fin>>n>>k;
    for (int i=1; i<=n; ++i)
        fin>>v[i];
    for (int i=2; i<=n; ++i)
        fact*=i;
    for (int i=1; i<=n; ++i){
        sol=v[i];
        bkt(i+1,2);
    }
    fout<<fixed<<setprecision(6)<<sum*2;
}