#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 calculeaza()
{
         p = 1;
     for(int d = 1 ; d <= k ;d++)
       p = p*c[v[d]];

      s += p;
}
void combinari(int l)
{
    if(l == k)
     j++,calculeaza();
     for(int i = v[l] + 1; i <= n;i++)
         {
           v[l+1] = i;
            combinari(l+1);
            }

}

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