#include <iostream>
#include <fstream>

using namespace std;

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

int n,k;
int sol[30];
int ap[30];
double v[30];
double sum;
int am;

void afis() {
    double c = 1;
    for(int i = 1; i <= k; i++)
        c *= v[sol[i]];
    sum += c;
    c = 1;
    for(int i = k; i >= 1; i--)
        c *= v[sol[i]];
    am++;
}

void bck(int top) {
    for(int i = sol[top-1]+1; i <= n; i++) {
        sol[top] = i;
        if(top == k)
            afis();
        else
            bck(top+1);
    }
}

int main() {

    in >> n >> k;

    for(int i = 1; i <= n; i++)
        in >> v[i];

    bck(1);

    out.precision(6);
    out << sum/am;

    return 0;
}