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

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

int ord[26], n, k;
double v[26], p, nr;

int main()
{
    f>>n>>k;
    for(int i=1;i<=n;i++)
        f>>v[i];
    for(int i=k+1;i<=n;i++)
            ord[i]=1;
    do
    {
        float x=1;
        for(int i=1;i<=n;i++)
            if(ord[i]==0)
            {
                x=x*v[i];
            }
        p+=x;
        nr++;
    }while(next_permutation(ord+1, ord+1+n));
    p=p/nr;
    g<<setprecision(6)<<p;
    return 0;
}