#include <bits/stdc++.h>
using namespace std;
int main() {
  long long x1, y1, x2, y2;
  cin >> x1 >> y1 >> x2 >> y2;
  unsigned long long res = llabs(x1 - x2) + 1;
  res *= llabs(y1 - y2) + 1;
  res /= 2;
  if ((y1 % 2 == 0 && y2 % 2 == 0) || (y1 % 2 != 0 && y2 % 2 != 0)) res++;
  cout << res << endl;
  return 0;
}
