#include <bits/stdc++.h>
using namespace std;
ifstream in("dezastru.in");
ofstream out("dezastru.out");
int  b[30] ,n,k,nr;
double a[30],sum,p;
int main()
{
     in >> n >> k;
     for(int i=1; i<=n; i++) in >> a[i],b[i] = i;

      do {
               nr++;
               p = 1.0;
             for(int i=1; i<=k; i++)
             {
                 p*=a[b[i]];
             }
      sum+=p;

     }while ( next_permutation(b+1, b+n+1) );
     out  << fixed << setprecision(7)<< sum / nr ;

    return 0;
}