#include <bits/stdc++.h>
using namespace std;
const int MAX = 1e9 + 7;
const int INF = 0x3f3f3f3f;
const long long MAXN = 1e5 + 7;
const long long MOD = 1e9 + 1;
int main() {
  long long x1, x2, y1, y2, x, y;
  cin >> x1 >> y1 >> x2 >> y2;
  x = (x2 - x1) >> 1;
  y = (y2 - y1) >> 1;
  cout << x * y + (x + 1) * (y + 1);
}
