#include <bits/stdc++.h>
using namespace std;
int main() {
  ios::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  long long x1, y1, x2, y2, le, le2;
  cin >> x1 >> y1 >> x2 >> y2;
  long long to = ((y2 - y1) / 2) + 1, to2 = to - 1;
  if ((x2 - x1) % 2 == 0) {
    le = ((x2 - x1) / 2) + 1;
    le2 = le - 1;
  } else {
    le = ((x2 - x1) / 2) + 1;
    le2 = le;
  }
  long long ans = (to * le) + (to2 * le2);
  cout << ans;
  return 0;
}
