#include <cstdio>
#include <algorithm>

using namespace std;

int main()
{
    int N,K,i,cnt=0,v[30];
    double p[30],s=0,prod;
    freopen ("dezastru.in","r",stdin);
    freopen ("dezastru.out","w",stdout);
    scanf("%d%d", &N,&K);
    for(i=1;i<=N;++i)
    {
        scanf("%lf", &p[i]);
        v[i]=i;
    }
    do
    {
        ++cnt;
        for(prod=i=1;i<=K;++i)
            prod*=p[v[i]];
        s+=prod;
    }
    while(next_permutation(v+1,v+N+1));
    s=s/cnt;
    printf("%.6lf\n", s);
    return 0;
}