#include <bits/stdc++.h>
using namespace std;
const long double pi = 3.1415926535897;
long long exp(long long base, long long power, int p) {
  if (!base) return 0;
  long long t = exp(base, power / 2, p);
  if (power & 1)
    return t * t * base % p;
  else
    return t * t % p;
}
void solve() {
  long long x1, y1, x2, y2, s;
  cin >> x1 >> y1 >> x2 >> y2;
  s = (y2 - y1) / 2 + 1;
  cout << (x2 - x1) / 2 * (s - 1) + ((x2 - x1) / 2 + 1) * s;
}
signed main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  int t = 1;
  while (t--) {
    solve();
    cout << "\n";
  }
  return 0;
}
