#include <bits/stdc++.h>
int main() {
  long long x1, x2, x3, x4, y1, y2, y3, y4, ver, hor;
  scanf("%lld %lld %lld %lld", &x1, &y1, &x3, &y3);
  x4 = x1;
  y4 = y3;
  x2 = x3;
  y2 = y1;
  ver = (y4 - y1) / 2;
  ver += 1;
  hor = (x2 - x1) / 2;
  long long tall = ver + (ver * hor);
  long long small = hor * (ver - 1);
  long long ans = tall + small;
  printf("%lld\n", ans);
  return 0;
}
