#include <bits/stdc++.h>

std::ifstream in("dezastru.in");
std::ofstream out("dezastru.out");

int n, t ;
double ans, curr, dp[40][40] ;

int main() {
  int i, j ;
  in >> n >> t ;
  dp[0][0] = 1 ;
  for (i = 1 ; i <= n ; ++ i) {
    in >> curr ;
    for (j = 1 ; j <= i ; ++ j) {
      dp[i][0] = 1 ;
      dp[i][j] = dp[i - 1][j] + curr * dp[i - 1][j - 1] ; } }
  ans = dp[n][t] ;
  for (i = t + 1 ; i <= n ; ++ i) {
    ans /= i ; }
  for (i = 1 ; i <= n - t ; ++ i) {
    ans *= i ; }
  out << ans ; }