#include <bits/stdc++.h>
using namespace std;
const long long INF = 1 << 30;
const long long LINF = 1 << 62;
const double EPS = 1e-9;
const long long MOD = 1e9 + 7;
int main() {
  pair<long long, long long> a, b;
  cin >> a.first >> a.second >> b.first >> b.second;
  long long ydif = (b.second - a.second + 2) / 2;
  long long xdif = b.first - a.first + 1;
  long long res = ydif * xdif;
  if (xdif % 2) {
    res -= xdif / 2;
  }
  cout << res << ("\n");
  return 0;
}
