#include <iostream>
#include <fstream>
#include <algorithm>

using namespace std;

ifstream f("dezastru.in");
ofstream g("dezastru.out");

int n;
long double v[100],s[700];
double probability;

int main()
{
    int k,count=0;
    f >>n>>k;
    for(int i=0;i<=700;++i){
        s[i]=1;
    }
    for(int i=0;i<n;++i){
        f >>v[i];
    }
    do{
        for(int i=0;i<k;++i){
            s[count]*=v[i];
        }
        count++;
    }while(next_permutation(v,v+n));
    probability=0;
    for(int i=0;i<count;++i){
        probability+=(s[i]/count);
    }
    g <<probability;
    return 0;
}