#include<iostream>
#include<vector>
#include<algorithm>

using namespace std;
using ll = long long;

using ld = long double;
#include<iomanip>
int main(){
    cin.tie(nullptr);
    ios::sync_with_stdio(false);

    ll n;
    cin>>n;
    vector<ll> a(n),r(n);
    for(int i = 0;i<n;i++) cin>>a[i];
    for(int i = 0;i<n;i++) cin>>r[i];

    ld ans = 0;
    for(int i = 0;i<n;i++){
        vector<long double> dp(n+1,0);
        dp[0] = 1;
        for(int j = 0;j<n;j++){
            if((j!=i&&a[j]<a[i])||(j<=i&&a[i]==a[j])){
                vector<long double> ndp(n+1,0);
                for(int k = 0;k<n;k++) ndp[k+1] += dp[k];
                swap(dp,ndp);
            }
            vector<long double> ndp(n+1,0);
            ndp[0] += dp[0];
            for(int k = 1;k<=n;k++){
                ndp[k] += dp[k] / 2.;

                if(k==1){
                    if(j<i) ndp[0] += dp[k] / 2.;
                    else{
                        ans += (long double) dp[k] * r[j] * a[i] / 2.;
                    }
                }else{
                    ndp[k-1] += dp[k] / 2.;
                }
            }
            swap(ndp,dp);
        }
        //cout<<ans<<endl;
    }
    cout<<setprecision(30)<<fixed<<ans<<endl;
}