#include <fstream>
#include <iomanip>

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

long double sum[26][26],nr[26][26],v[26];
int n,k;

int main()
{
    in>>n>>k;
    for(int i=1;i<=n;i++)
    {
        in>>v[i];
    }
    sum[0][0]=1;
    nr[0][0]=1;
    for(int i=1;i<=n;i++)
    {
        sum[i][0]=nr[i][0]=1;
        for(int j=1;j<=n;j++)
        {
            sum[i][j]=sum[i-1][j-1]*v[i]+sum[i-1][j];
            nr[i][j]=nr[i-1][j-1]+nr[i-1][j];
        }
    }
    out<<fixed<<setprecision(6)<<sum[n][k]/nr[n][k];
    return 0;
}