#include <iostream>

#include <cstdio>

#include <cmath>

#include <algorithm>

using namespace std;

double state[(1<<20)+10];

double ps[25];

int main(){

	int n; double noted,is;

	while(scanf("%d",&n)!=EOF){

		double fail=1;

		for(int i=0;i<n;i++){

			scanf("%lf",&ps[i]);

			fail-=ps[i];

		}

		int al=(1<<n)-1;

		for(int i=al;i>=0;i--){

			if(i==al){

				state[i]=0;

				continue;

			}

			noted=fail;is=1;

			for(int k=0;k<n;k++){

				if(i&(1<<k)){

					noted+=ps[k];

				}

				else{

					is+=(ps[k]*state[i|(1<<k)]);

				}

			}

			state[i]=is/(1-noted);

		}

		printf("%lf\n",state[0]);

	}

	return 0;

}