#include <iostream>
#include <fstream>
#include <iomanip>

using namespace std;


// dp[i][j] din primele i am ales j

int n, k, nr;


double calcul_cst ( int n, int k ){
  double ans1 = 1, ans2 = 1;
  for ( int i = 1; i <= k; i++ ){
    ans1 *= (n - k + i);
    ans2 *= i;
  }
  return ans1 / ans2;
}


int main (){

  ifstream in ("dezastru.in");
  in >> n >> k;
  double p[n + 2];
  double dp[n + 1][n + 1] = {};
  for (int i = 1; i <= n; i++ )
    in >> p[i];
  in.close();

  for (int i = 0; i <= n; i++ )
    dp[i][0] = 1.0;

  // for ( int i = 0; i < 3;i ++ ){
  //   for ( int j = 0; j < 3; j ++ )
  //     cout << dp[i][j] << " ";
  //   cout << '\n';
  // }


  double MOD = calcul_cst(n, k);
  for ( int  i = 1; i <= n; i++ )
    for ( int j = 1; j <= k; j++ )
      dp[i][j] = dp[i - 1][j] + (dp[i - 1][j - 1] * p[i]);


  ofstream out ("dezastru.out");
  out << setprecision(6) << dp[n][k] / MOD;
  out.close();

  return 0;
}