#include <bits/stdc++.h>
using namespace std;
const long long INF = LLONG_MAX;
const double PI = acos(-1);
const long long MOD = 1e9 + 7;
const long long N = 1e5 + 10;
void solve();
int main() {
  ios_base ::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  solve();
  return 0;
}
void solve() {
  long long x1, x2, y1, y2;
  cin >> x1 >> y1 >> x2 >> y2;
  long long lol = (y2 - y1 + 2) / 2;
  long long xD = x2 - x1 + 1;
  long long haha = (x2 - x1) / 2;
  cout << lol * xD - haha << "\n";
  return;
}
