#include <iostream>
#include <cstdio>
#include <algorithm>
#include <cstring>
#include <queue>
#include <cmath>
#include <vector>
#include <iomanip>
#define ld double
using namespace std;
int read(){
	int x=0,f=1;char ch = getchar();
	while(ch<'0'||ch>'9'){if(ch=='-'){f=-1;}ch = getchar();}
	while(ch>='0'&&ch<='9'){x = x*10+ch-'0';ch = getchar();}
	return x*f;
}
const int N = 25;
ld dp[(1<<20)+5];
int n,l[N],w[N];
int main(){
	n = read();
	for(int i=0;i<n;i++){
		l[i] = read();
	}
	for(int i=0;i<n;i++){
		w[i] = read();
	}
	int up = (1<<n);
	for(int i=0;i<up;i++){
		int sum = 0;
		for(int j=0;j<n;j++){
			if((i>>j)&1){
				sum+=w[j];
			}
		}
		for(int j=0;j<n;j++){
			if(!((i>>j)&1)){
				dp[i^(1<<j)] = max(dp[i^(1<<j)],(ld)dp[i]+l[j]*0.5*w[j]/(w[j]+sum));//当前块重心在里面 
				dp[i^(1<<j)] = max(dp[i^(1<<j)],(ld)l[j]*0.5+l[j]*0.5*sum/(w[j]+sum));//当前块重心在外面 
			}
		}
	}
	cout<<fixed<<setprecision(20)<<dp[up-1]<<"\n"; 
//	cout<<dp[up-1]<<"\n";
	return 0;
}