#include <bits/stdc++.h>
#pragma comment(linker, "/STACK:100000000")
using namespace std;
int main() {
  long long x1, x2, y1, y2;
  scanf("%lld%lld%lld%lld", &x1, &y1, &x2, &y2);
  long long r = ((y2 - y1) / 2) * ((x2 - x1) / 2) +
                ((y2 - y1) / 2 + 1) * ((x2 - x1) / 2 + 1);
  printf("%lld\n", r);
  return 0;
}
