#include <iostream>

using namespace std;

double sum,a[30];
int tot,n,k,i;

void comb(int add,int x,double prod)
{
    if(!add)
    {
        sum+=prod;
        ++tot;
        return;
    }
    for(; x<=n-add+1; ++x)
       comb(add-1, x+1, prod*a[x]);
}

int main()
{
    freopen("dezastru.in", "r", stdin);
    freopen("dezastru.out", "w", stdout);

    scanf("%d%d", &n, &k);

    for(i=1; i<=n; ++i) scanf("%lf", &a[i]);

    comb(k,1,1);

    printf("%.6lf\n", sum/tot);

    return 0;
}