#include <fstream>
#include <iomanip>
#include <iostream>

using namespace std;

ifstream fin("dezastru.in");
ofstream fout("dezastru.out");

int n, m, x[30], cnt;
double p[30], sum, odd = 1;

void bck(int k)
{
    if (k == m+1)
    {
        sum+=odd;
        cnt++;
    }
    else
    {
        for (int i = x[k-1]+1; i <= n-m+k; i++)
        {
            x[k] = i;
            odd*=p[i];
            bck(k+1);
            odd/=p[i];
        }
    }
}
int main()
{
    fin >> n >> m;
    for (int i = 1; i <= n; i++)
        fin >> p[i];
    bck(1);
    fout << fixed << setprecision(6) << sum/cnt;
    return 0;
}