#include <fstream>
#include <algorithm>
#include <iomanip>
#define nrmax 26
using namespace std;
ifstream input("dezastru.in");
ofstream print("dezastru.out");
int n,k,nr;
double V[nrmax*nrmax],sum,x;

void citire(){
    input>>n>>k;
    for(int i=0;i<n;i++)
        input>>V[i];
    input.close();
}

void rezolvare(){
    do{ x=1;
        for(int i=0;i<k;i++)
        x*=V[i];
        sum+=x;
        nr++;
    }while(next_permutation(V,V+n));
}

int main(){
    citire();
    rezolvare();
    print<<fixed<<setprecision(6)<<sum/nr;
    return 0;
}