#include <bits/stdc++.h>
using namespace std;
ifstream fin("dezastru.in");
ofstream fout("dezastru.out");
int n,k,a[30];
float v[30];
float p,total;
unsigned long long fct=1;
void produs()
{
    p=1;
    for (int i=1;i<=k;i++)
        p*=v[a[i]];
    total+=p;
}
int main()
{
    fin >> n >> k;
    for (int i=1;i<=n;i++)
        fct=fct*i;
    for (int i=1;i<=n;i++)
        {
            fin >> v[i];
            a[i]=i;
        }
    do
    {
        produs();
    }while(next_permutation(a+1,a+n+1));
    total/=fct;
    fout<<total<<'\n';
    return 0;
}