#include <iostream>
#include <cstdio>
using namespace std;

const int N_max = 27;

double p[N_max];

int sol[N_max];

double P;
int NR;

int N, K;

void write()
{
    double pc = 1;

    for(int i = 1; i <= K; i++) pc = pc * p[ sol[i] ];

    P += pc;
}

void bkt(int p)
{
    if(p - 1 == K)
    {
        NR++;
        write();
    }
    else
        for(int i = 1 + sol[p - 1]; i <= N - K + p; i++)
        {
            sol[p] = i;
            bkt(p + 1);
        }
}

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

    scanf("%d %d", &N, &K);
    for(int i = 1; i <= N; i++) scanf("%lf", &p[i]);

    bkt(1);

    printf("%.6lf", P / NR);

    return 0;
}