#include <bits/stdc++.h>
using namespace std;
long double bin_pow(long long a, long long c) {
  if (c == 0) return 1;
  if (c % 2 == 0)
    return bin_pow(a * a, c / 2);
  else
    return a * bin_pow(a, c - 1);
}
int main() {
  long long x1, x2, y1, y2, ny1, ny2;
  cin >> x1 >> y1 >> x2 >> y2;
  ny1 = y1 - 1;
  ny2 = y2 + 1;
  cout << (x2 - x1) / 2 * (ny2 - ny1 + y2 - y1) / 2 + (ny2 - ny1) / 2;
  return 0;
}
