#include <bits/stdc++.h>
using namespace std;
int main() {
  long long y2, y1, x2, x1;
  cin >> x1 >> y1 >> x2 >> y2;
  printf("%lld\n",
         ((long long)fabs(y2 - y1) / 2 + 1) * ((long long)fabs(x2 - x1) + 1) -
             (long long)fabs(x2 - x1) / 2);
  return 0;
}
