#include <bits/stdc++.h>
using namespace std;
int main() {
  ios::sync_with_stdio(!cin.tie(0));
  long long x1, y1, x2, y2;
  cin >> x1 >> y1 >> x2 >> y2;
  x2 -= x1;
  y2 -= y1;
  x1 = 0, y1 = 0;
  long long xo, xe, oo, ee;
  if (x1 % 2) {
    xe = (x2 - x1) / 2;
    xo = xe + 1;
  } else {
    xo = (x2 - x1) / 2;
    xe = xo + 1;
  }
  if (y1 % 2) {
    oo = (y2 - y1) / 2 + 1;
    ee = (y2 - y1) / 2;
  } else {
    oo = (y2 - y1) / 2;
    ee = (y2 - y1) / 2 + 1;
  }
  cout << xo * oo + xe * ee << '\n';
}
