#include <bits/stdc++.h>
using namespace std;
#define rep(i,n) for(int i = 0; i < (int)n; i++)
using ll = long long;
constexpr double eps = 1e-8;

int main(){
  int n;
  cin >> n;
  vector<ll> a(n), r(n), b(n+1), id(n);
  rep(i,n) cin >> a[i];
  rep(i,n) cin >> r[i];
  b[0] = 0;
  rep(i,n) b[i+1] = a[i];
  sort(b.begin(),b.end());
  b.erase(unique(b.begin(),b.end()),b.end());
  int m = b.size();
  map<int,int> mp;
  rep(i,m) mp[b[i]] = i;
  rep(i,n) id[i] = mp[a[i]];
  vector<vector<double>> dp(m+1,vector<double>(n+1));
  rep(j,id[0]) dp[j][0] = 1.0;
  for(int j = id[0]; j < m; j++) dp[j][1] = 1.0;
  double ans = 0.0;
  rep(i,n) {
    for(int j = 1; j <= m; j++) ans += dp[j-1][0]*r[i]*(b[j]-b[j-1])/2;
    vector<vector<double>> dpn(m+1,vector<double>(n+1));
    if(i == n-1) break;
    rep(j,m+1) {
      int nk = 0;
      if(j >= id[i+1]) nk++;
      dpn[j][nk] += dp[j][0];
    }
    rep(j,m+1) for(int k = 1; k <= n; k++) {
      if(dp[j][k] < eps) continue;
      for(int df = -1; df <= 0; df++) {
        int nk = k+df;
        if(j >= id[i+1]) nk++;
        if(nk >= 0 and nk <= n) dpn[j][nk] += dp[j][k]/2.0;
      }
    }
    swap(dp,dpn);
  }
  cout << fixed << setprecision(12) << ans << endl;
  return 0;
}