#include <bits/stdc++.h>
using namespace std;
int main() {
  long long int a, b, c, d, x, y, l = 0;
  cin >> a >> b >> c >> d;
  x = c - a;
  y = d - b;
  l = ((x / 2) * (y + 1)) + (y / 2) + 1;
  cout << l;
  return 0;
}
