#include <bits/stdc++.h>
using namespace std;
int main() {
  long long int x1, y1, x2, y2;
  cin >> x1 >> y1 >> x2 >> y2;
  long long int Y = abs(y2 - y1);
  Y /= 2;
  Y++;
  long long int X = abs(x1 - x2) + 1;
  long long int ans = X * Y;
  ans -= (X / 2);
  cout << ans;
}
