#include <bits/stdc++.h>
using namespace std;
int a, b, c, d;
int main() {
  scanf("%d%d%d%d", &a, &b, &c, &d);
  int n = c - a + 1;
  int m = (d - b) / 2 + 1;
  printf("%lld\n", 1LL * n * m - n / 2);
  return 0;
}
