#include<bits/stdc++.h>
using namespace std;
int n;
double dp[3005][3005],a[3005],ans=0.0;
int main()
{
	cin>>n;
	for(int i=1;i<=n;i++)cin>>a[i];
	dp[0][0]=1;
	for(int i=1;i<=n;i++)for(int j=0;j<=i;j++)
		dp[i][j]=dp[i-1][j]*(1-a[i])+dp[i-1][j-1]*a[i];
	for(int i=n/2+1;i<=n;i++)ans+=dp[n][i];
	cout<<fixed<<setprecision(9)<<ans;
	return 0;
}