#include <bits/stdc++.h>
using namespace std;
int gcd(int a, int b) {
  if (a == 0 || b == 0) return 0;
  if (a == b) return a;
  if (a > b) return gcd(a - b, b);
  return gcd(a, b - a);
}
int lcm(int a, int b) { return (a * b) / gcd(a, b); }
int a[1501];
int main() {
  long long int x1, x2, y1, y2;
  cin >> x1 >> y1 >> x2 >> y2;
  x2 -= x1;
  y2 -= y1;
  cout << (((y2 / 2) + 1) * ((x2 / 2) + 1)) + ((x2 * y2) / 4);
}
