#include <bits/stdc++.h>
#define int long long int

using namespace std ;

const int N = 4000 ;
int n , a , b[3];
long double d[N] , dp[N][N];

int32_t main(){
	cout << setprecision(30) << fixed ;
	cin >> n ;
	for(int i = 0 ; i < n ; i++){
		cin >> d[i] ;//head
	}
	dp[0][0] = 1 - d[0] ;
	dp[1][0] = d[0] ;
	for(int i = 1 ; i < n ; i++){
		for(int j = 0 ; j <= n ; j++){			
			dp[j][i] = 1 - d[i] ;
			dp[j][i] *= dp[j][i - 1] ;
			if(j)dp[j][i] += d[i] * dp[j - 1][i - 1] ;
		}
	}
	long double ans = 0 ;
	for(int i = n / 2 + 1 ; i <= n ; i++){
		ans += dp[i][n - 1] ;
	}
	cout << ans ;
	return 0 ; 
}
/*
#include <radman/roderc++.h>
                     ....._____
                 .~~~~~////////\\                                                                                           ...
              .:::::////////////\\                                          ....::////////..                              (/\\\\...
           .::::://////   \\\\///\\              .:::://                ..:::::///////////////.                           \\\\\$$$$$$
         \\\\\//////      \\\\///\\           //\\\//////             \\\\\////////////////////.                           ""\\$$$$$$$
         \\\\\////         \\\\////         |||||////////.            \\\\\/////     ~~~~~//////          ...                  "\$$$$
         \\\\\//\         ////|||/          \\\\\/////////            \\\\\//\       ~~~~~//////        (/\\\\...                   .
         \\\\\//\        ////|||/          |||||////*\\///.           \\\\\//\       ~~~~~//////        \\\\\$$$$$$                 .@@@@@.
         \\\\\//\       ////|||/           \\\\\///|\\\////           \\\\\//\       ~~~~~//////         ""\\$$$$$$$               @@@@////\
         \\\\\//\ ____---/|||/            |||||///|\\\\\///.          \\\\\//\       ~~~~~//////             "\$$$$                @@@@\///\
         \\\\\//\--|||||||_-'             \\\\\///|\\\\\////          \\\\\//\       ~~~~~//////                                  ,@@@@|///|
         \\\\\//\|||||_"-\\\\\           |||||///|  ~~~~~///.         \\\\\//\      ~~~~~//////     .@@@@.                    /@@@@@|/////
         \\\\\//\///"""\\\\\\\\          \\\\\///|~~~~[[[////.        \\\\\//\     ~~~~~//////      @@@@@@@@                ..@@@@@.//////
         \\\\\//\    \"""\\\\\\\        |||||///|~~[[[[["\////.       \\\\\//\    ~~~~~//////      '@@@@/////\       .:;@@@@@@@..///////"
         \\\\\//\      \"""\\\\\\       \\\\\///|[[[[*\\\\\////.      \\\\\//\   ~~~~~//////        *@@@\//////\@@@@@@@@..//////////"
         \\\\\//\       \:::\\\\/      |||||///|[[[*   \\\\\\///      \\\\\//\~~~~~///////'            *@@\/////////////////////"
         \\\\\//\        \___\\/       \\\\\///|*       \\\\\/"       \\\\\//\~~////////'                 **"///////////"""
         \\\\\//\                     |||||///|                       \\\\\//\///////"
         \\\\\//\                     \\\\\///|                       \\\\\//\////"
         \\\\\///                    |||||////|                       \\\\\////"
         \\\\\/"                     \\\\\///*                        \\\\\/"
                                     """""*
*/


Measured memory_bytes: 188288