#include<bits/stdc++.h>
#define double long double
using namespace std;
inline int read(){
	int ans=0;bool op=0;char ch=getchar();
	while(ch<'0'||'9'<ch){if(ch=='-')op=1;ch=getchar();}
	while('0'<=ch&&ch<='9'){ans=(ans<<1)+(ans<<3)+(ch^48);ch=getchar();}
	if(op)return -ans;
	return ans;
}
const int maxn=20;
int n;
double dp[1<<maxn];
int l[maxn],w[maxn];
int gsum(int x){
	int ans=0;
	for(int i=0;i<n;i++)if(x>>i&1)ans+=w[i];
	return ans;
}
signed main(){
	n=read();
	for(int i=0;i<n;i++)l[i]=read();
	for(int i=0;i<n;i++)w[i]=read();
	dp[0]=0;
	for(int i=0;i<(1<<n);i++){
		for(int k=0;k<n;k++){
			if(i>>k&1){
				int j=i^(1<<k);
				int sm=gsum(j);
				dp[i]=max(dp[i],w[k]*0.5*l[k]/(sm+w[k])+dp[j]);
				dp[i]=max(dp[i],(double)0.5*l[k]*sm/(sm+w[k])+0.5*l[k]);
			}
		}
	}
	cout<<fixed<<setprecision(20)<<dp[(1<<n)-1];
	return 0;
}