//一時保存

#include "bits/stdc++.h"
using namespace std;
#define int long long
#define endl '\n'
int mod=1e9+7;
int mod2=998244353;
const int INF=1e9;

signed main(){
	int n;
	cin>>n;
	vector<double> p(n);
	for(int i=0;i<n;i++)cin>>p[i];
	vector<vector<double> > dp(n+1,vector<double>(n+1,0));
	dp[0][0]=1;
	for(int i=0;i<n;i++){
		for(int j=0;j<n;j++){
			dp[i+1][j]+=dp[i][j]*(1-p[i]);
			dp[i+1][j+1]+=dp[i][j]*p[i];
		}
	}
	double ans=0;
	for(int i=n/2+1;i<=n;i++)ans+=dp[n][i];
	//if(n%2==1)ans+=dp[n][n/2];
	cout<<fixed<<setprecision(20)<<ans<<endl;
}

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