#include<bits/stdc++.h>
#define rp(a,b,c) for(long long a=(b);a<=(c);a++)
#define FILL(x,v) memset(x,v,sizeof(x))

using namespace std;
typedef long long ll;

ll n;
double p[20+5];
double ans[1<<25];
double faceup[1<<25];
double facedown[1<<25];

double dfs(ll ST)
{
	if(ans[ST]>=0) return ans[ST];
	double psum=0.0;
	double esum=0.0;
	rp(i,0,n-1)
	{
		if(ST&(1<<i))
		{
			ll afterclick = (ST ^ (1<<i));
			double curP = faceup[1<<i] * facedown[afterclick] + facedown[1<<i] * faceup[afterclick];
			esum+=curP*dfs(afterclick);
			psum+=curP;
		}
	}
	
	return ans[ST] = (1+esum)/psum;
}

int main()
{
	cin>>n;
	rp(i,0,n-1)
		cin>>p[i];
	rp(i,0,(1<<n)-1)
	{
		ans[i]=-1;
	}
	rp(i,0,n-1)
	{
		rp(j,i+1,n-1)
		{
			ans[(1<<i)|(1<<j)]=0;
		}
	}
	
	
	
	rp(i,1,(1<<n)-1)
	{
		faceup[i]=1;
		facedown[i]=1;
	}
	
	
	rp(i,0,n-1)
	{
		rp(stat,1,(1<<n)-1)
		{
			if(stat&(1<<i))
			{
				faceup[stat] *= p[i];
				facedown[stat] *= (1-p[i]);
			}
		}
	}

	
	
	dfs((1<<n)-1);
	printf("%.7lf\n",ans[(1<<n)-1]);
	
	return 0;
}