#include <bits/stdc++.h>
using namespace std;
int main() {
  long long int i, j, l, a, d, n, m = 0, b, c, s, k;
  long long int x1, x2, y1, y2, ans;
  cin >> x1 >> y1 >> x2 >> y2;
  a = x2 - x1 + 1;
  b = y2 - y1;
  if (b % 2 == 0) {
    c = b / 2 + 1;
    s = b / 2;
  } else {
    c = b / 2 + 1;
    s = b / 2 + 1;
  }
  ans = (a - a / 2) * c + (a / 2) * s;
  cout << ans;
  return 0;
}
