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

using namespace std;

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

float v[30],a[30][30],b[30][30];


int main()
{
    int n,k;
    f >> n >> k;
    for(int i = 1; i <= n; i++)
        f >> v[i];
    a[0][0] = b[0][0] = 1;
    for(int i = 1; i <= n; i++)
    {
        a[i][0] = b[i][0] = 1;
        for(int j = 1; j <= n; j++)
        {
            a[i][j] = a[i-1][j] + a[i-1][j-1] * v[i];
            b[i][j] = b[i-1][j] + b[i-1][j-1];
        }
    }
    g << fixed << setprecision(6) << a[n][k] / (float) b[n][k];
    return 0;
}