#include <iostream>
#include <fstream>
using namespace std;
ifstream I ("dezastru.in");
ofstream O ("dezastru.out");
int n,l , comb[25];
double s, j, p;
double val[25];
void back (int k);
int main () {
    I>>n>>l;
    for (int i=1;i<=n;i++) 
        I>>val[i];
    back(1);
    O<<s/j;
    return 0;
}
void back (int k) {
     int i;
     for (i=1;i<=n;i++) {
         if (comb[k-1]<i) {
             comb[k]=i;
             if (k==l) {
                p=1;
                for (int i=1;i<=l;i++)
                   p=p*val[comb[i]];
                s=s+p;
                j=j+1;
             }       
             else
                 back(k+1);
         }                     
     }
}