#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;
bool sol[30];
void permutari(int l)
{
    if(l > k)
     j++,s+=p;
     else
     for(int i = 1; i <= n;i++)
     {
         if(!sol[i])
         {
            sol[i] = true;
            v[l] = i;
            p*=c[v[l]];
            permutari(l+1);
            sol[i] = false;
            p/=c[v[l]];
            }
     }
}
int main()
{
    int i;
   f>>n>>k;
   for(i = 1; i <= n ;i++)
    f>>c[i];
   permutari(1);
     s = s/j;
   g<<float(s);
    return 0;
}