#include <bits/stdc++.h>

using namespace std;
ifstream f("dezastru.in");
ofstream g("dezastru.out");

int N, K, P[26],nrp;
double pr[26],v,PR;

int main()
{
    int i;
    //citire
    f>>N>>K;
    for(i = 1; i <= N; i++)f>>pr[i];
    //generare permutari
    for(i = 1; i <= N; i++) P[i] = i;
    do
    {
       v=1;
       //calculam probab sa suprav primelor k atacuri
       for(i=1;i<=K;i++)v=v*pr[P[i]];
       PR=PR+v;
       nrp++;
    }
    while(next_permutation(P + 1, P + N + 1));

    //afisare
    g.precision(6);
    g<<fixed<<PR/nrp;
    return 0;
}