#include<bits/stdc++.h>
#define fi first
#define se second
#define ull unsigned long long
#define make make_pair
#define N 21
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;
}
int n,l[N],w[N];
double f[1<<N];
signed main()
{
	n=read();
	for(int i=0;i<n;i++)l[i]=read();
	for(int i=0;i<n;i++)w[i]=read();
	f[0]=0;int lim=(1<<n)-1;
	for(int i=0;i<=lim;i++)
	{
		int sum=0;
		for(int j=0;j<n;j++)if(i&(1<<j))sum+=w[j];
		for(int j=0;j<n;j++)
		{
			if(i&(1<<j))continue;
			double k=w[j]*l[j]*1.0/(2.0*(sum+w[j])); 
			f[i|(1<<j)]=max(f[i|(1<<j)],f[i]+k);
			f[i|(1<<j)]=max(f[i|(1<<j)],l[j]*1.0-k);
		}
	}
	printf("%.10f",f[lim]);
	return 0;
}