#include <iostream>
#include <cstdio>
#include <iomanip>

using namespace std;

int n, k, viz[30], sol[30], imp;
float a[30], rasp=0, raspAjutor;

void citire()
{
    scanf("%d %d \n", &n, &k);
    for(int i=1; i<=n; ++i)
        scanf("%f ", &a[i]);
}
void rezolvare()
{
    raspAjutor=1;
    for(int i=1; i<=k; ++i)
        raspAjutor*=a[sol[i]];
    rasp+=raspAjutor;
}
void back(int m)
{
    if(m>=k+1)
    {
        rezolvare();
        imp++;
        return;
    }
    for(int i=1; i<=n; ++i)
        if(!viz[i])
        {
            sol[m]=i;
            viz[i]=1;
            back(m+1);
            viz[i]=0;
        }
}
int main()
{
    freopen("dezastru.in", "r", stdin);
    freopen("dezastru.out", "w", stdout);
    citire();
    back(1);
    printf("%f", rasp/imp);
    return 0;
}