#include <bits/stdc++.h>
using namespace std;
int main(){
  cout << fixed << setprecision(20);
  int N;
  cin >> N;
  vector<int> A(N), R(N);
  for (int i = 0; i < N; i++){
    cin >> A[i];
  }
  for (int i = 0; i < N; i++){
    cin >> R[i];
  }
  double ans = 0;
  for (int i = 0; i < N; i++){
    vector<vector<double>> dp(N + 1, vector<double>(N + 1, 0));
    dp[0][0] = 1;
    for (int j = 0; j < N; j++){
      int d = 0;
      if (A[j] < A[i] || A[j] == A[i] && j < i){
        d = 1;
      }
      vector<double> tmp(N + 1, 0);
      for (int k = 0; k <= j; k++){
        tmp[k + d] = dp[j][k];
      }
      if (j >= i){
        ans += A[i] * R[j] * tmp[0] / 2;
      } else {
        dp[j + 1][0] += tmp[0] / 2;
      }
      for (int k = 1; k <= N; k++){
        dp[j + 1][k - 1] += tmp[k] / 2;
      }
      for (int k = 0; k <= N; k++){
        dp[j + 1][k] += tmp[k] / 2;
      }
    }
  }
  cout << ans << endl;
}