#include <fstream>
#include <vector>
#include <algorithm>
#include <iomanip>
using namespace std;

ifstream be("dezastru.in");
ofstream ki("dezastru.out");

int n,k;
float x[30],b[30],db;

int main()
{
    be>>n>>k;

    for(int i=1;i<=n;i++)
        be>>x[i];
    vector<int>v;
    for(int i=1;i<=n;i++)
        v.push_back(i);
    b[0]=1;
    int m=0;
    do{

         for(int i=1;i<=k;i++){

            b[i]=b[i-1]*x[v[i]];
         }
        m++;
        db+=b[k];

    }while(next_permutation(v.begin(),v.end()));

    ki<<setprecision(6)<<db/m<<endl;
    return 0;
}