#include <bits/stdc++.h>
using namespace std;
long long _, __, ___, ____;
int main() {
  while (~scanf("%lld%lld%lld%lld", &_, &___, &__, &____)) {
    long long NOIP = __ - _ + 1, NOI = ____ - ___ + 1;
    printf("%lld\n",
           (NOIP - NOIP / 2) * (NOI - NOI / 2) + (NOIP / 2) * (NOI / 2));
  }
  return 0;
}
