#include <stdio.h>
#include <bits/stdc++.h>
#include <atcoder/all>
using namespace atcoder;
using mint = modint998244353;
using namespace std;
#define rep(i,n) for (int i = 0; i < (n); ++i)
#define Inf 1000000001

int N;
vector<int> A,R;

vector<double> get(int n){
	vector<double> dp(N+1,0.0);
	dp[0]  = 1.0;
	vector<double> ret(N,0.0);
	
	rep(i,N){
		vector<double> ndp(N+1,0.0);
		rep(j,N+1){
			if(A[i]<=n){
				if(i!=N){
					ndp[j+1] += dp[j];
				}
			}
			else{
				ndp[j] += dp[j];
			}
		}
		
		swap(dp,ndp);
		ndp.assign(N+1,0.0);
		rep(j,N+1){
			if(j==0)continue;
			ret[i] += dp[j]/2.0;
		}
		rep(j,N+1){
			if(j==0){
				ndp[j] += dp[j];
			}
			else{
				ndp[j] += dp[j]/2.0;
				ndp[j-1] += dp[j]/2.0;
			}
		}
		swap(dp,ndp);		
	}
	return ret;
}
	
int main(){
	
	cin>>N;
	
	A.resize(N),R.resize(N);
	rep(i,N)cin>>A[i];
	rep(i,N)cin>>R[i];
	
	vector<int> t = A;
	t.push_back(0);
	sort(t.begin(),t.end());
	t.erase(unique(t.begin(),t.end()),t.end());
	
	vector<double> x = get(t[0]);
	double ans = 0.0;
	for(int i=1;i<t.size();i++){
		vector<double> y = get(t[i]);
		/*
		cout<<t[i]<<endl;
		rep(j,N){
			cout<<y[j]<<',';
		}
		cout<<endl;
		*/
		rep(j,N){
			ans += (y[j]-x[j]) * t[i] * R[j];
		}
		
		swap(x,y);
	}
	
	cout<<fixed<<setprecision(10)<<ans<<endl;
	
	return 0;
}