#include<bits/stdc++.h>

using namespace std;
const int N = 3000;
double dp[N][N],p[N];
int main(){
	int n;
	cin>>n;
	for(int i=1;i<=n;i++) cin>>p[i];
	dp[0][0]=1;
	for(int i=1;i<=n;i++){
		for(int j=1;j<=i;j++){
			dp[i][j]+=dp[i-1][j-1]*p[i];
			dp[i][j-1]+=dp[i-1][j-1]*(1-p[i]);
		}
	}
	double ans=0;
	for(int i=n/2+1;i<=n;i++){
		ans+=dp[n][i];
	}
	//cout<<dp[3][3]<<endl; 
	printf("%.10lf\n",ans);
	return 0;
}