#include <bits/stdc++.h>
using namespace std;
int main() {
  long long int x1, x2, y1, y2;
  cin >> x1 >> y1 >> x2 >> y2;
  long long int sum = 0;
  long long int x = x1, y = y1;
  long long int w = x2 - x1 + 1, h = 1;
  while (y < y2) {
    y += 2;
    h++;
  }
  sum += (h - 1) * w;
  if (y == y2) {
    sum += w / 2;
    if ((x1 % 2 + 2) % 2 == (x2 % 2 + 2) % 2) sum++;
  }
  cout << sum << endl;
  return 0;
}
