#include <bits/stdc++.h>
using namespace std;
struct _nl {
} nl;
bool _ns = 0;
template <typename T>
void _p(const T& _) {
  if constexpr (is_same<T, _nl>()) {
    cout << "\n";
    _ns = 0;
  } else {
    if (_ns) cout << " ";
    cout << _;
    _ns = 1;
  }
}
template <typename... T>
void out(const T&... _) {
  (_p(_), ...);
}
int ii() {
  int _;
  cin >> _;
  return _;
}
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  long long x1 = ii(), y1 = ii(), x2 = ii(), y2 = ii();
  long long dx = x2 - x1;
  long long dy = y2 - y1;
  long long yy = dy / 2 + 1;
  long long xx = (dx + 1) / 2;
  long long ans = xx * (yy * 2 - 1);
  if (!(dx & 1)) {
    ans += yy;
  }
  out(ans, nl);
  return 0;
}
