#include <iostream>
#include <fstream>
using namespace std;
ifstream f("dezastru.in");
ofstream g("dezastru.out");
int dp[32],N,K,nrSol;
double v[32],sum;
void Bkt(int k,double Sol)
{
  for(int i=dp[k-1]+1;i<=N;i++)
   {
    dp[k]=i;
    if(k==K)
    {
      nrSol++;
      sum+=Sol*v[dp[k]];
    }
    else Bkt(k+1,Sol*v[dp[k]]);
}
}
int main()
{
    f>>N>>K;
    for(int i=1;i<=N;i++)
      f>>v[i];

    Bkt(1,1);
    g<<(double)sum/nrSol;
    return 0;
}