#include <iostream>
#include <cmath>
#include <algorithm>
using namespace std;
typedef long double ld;
int a[1010];
ld r[1010],dp[510][510] = {};
int main(){
    cout.precision(20);
    int i,j,k,n; cin >> n;
    for(i=0;i<n;i++) cin >> a[i];
    for(i=0;i<n;i++) cin >> r[i];
    ld ans = 0;
    for(int val = 1;val<=1000;val++){
        for(i=0;i<=n;i++){
            for(j=0;j<=n + 1;j++) dp[i][j] = 0;
        }
        dp[0][0] = 1;
        for(i=1;i<=n;i++){
            for(j=0;j<=i;j++){
                if(a[i - 1]<val){
                    dp[i][j] = dp[i - 1][j]/2;
                    if(j>0) dp[i][j] += dp[i - 1][j - 1]/2;
                }else{
                    if(j>0) dp[i][j] = dp[i - 1][j]/2 + dp[i - 1][j + 1]/2;
                    else dp[i][j] = dp[i - 1][j] + dp[i - 1][j + 1]/2;
                }
            }
        }
        for(i=1;i<=n;i++){
            if(a[i - 1]>=val) ans += r[i - 1]*dp[i - 1][0]/2;
        }
    }
    cout << ans << endl;
}