#include <bits/stdc++.h>
using namespace std;
int main(void) {
  int x1, x2, y1, y2;
  scanf("%d%d%d%d", &x1, &y1, &x2, &y2);
  long long x = (x2 - x1) / 2, y = (y2 - y1) / 2;
  long long ans = (x + 1) * (y + 1) + x * y;
  printf("%I64d\n", ans);
  return 0;
}
