#include <iostream>
#include <fstream>
#include <iomanip>
using namespace std;
ifstream f("dezastru.in");
ofstream g("dezastru.out");
int v[30],n,j,k;
float c[30],s = 0,p =1;
void combinari(int l)
{
    if(l == k)
     j++,s=s+p;
     for(int i = v[l] + 1; i <= n;i++)
            {

                 v[l+1] = i;
                 p*=c[i];
            combinari(l+1);
               p/=c[i];
            }

}

int main()
{
    int i;
   f>>n>>k;
   for(i = 1; i <= n ;i++)
    f>>c[i];
   combinari(0);
     s = s/j;
   g<<setprecision(6)<<s;
    return 0;
}