#include <bits/stdc++.h>
using namespace std;
const int inf = 1e9 + 5;
const int nax = 1e6 + 5;
void f(long long low, long long high, long long& even, long long& odd) {
  long long M = 1000LL * 1000 * 1000 * 1000 + 4;
  low += M;
  high += M;
  even = high / 2 - (low - 1) / 2;
  odd = high - low + 1 - even;
}
int main() {
  long long x1, y1, x2, y2;
  scanf("%lld%lld%lld%lld", &x1, &y1, &x2, &y2);
  const long long K = 1000 * 1000 * 1000;
  assert(-K <= x1 && x1 <= x2 && x2 <= K);
  assert(-K <= y1 && y1 <= y2 && y2 <= K);
  assert((y2 - y1) % 2 == 0);
  assert((x2 - x1) % 2 == 0);
  long long half = (x2 - x1) / 2;
  long long h2 = (y2 - y1) / 2;
  printf("%lld\n", half * h2 + (half + 1) * (h2 + 1));
  return 0;
}
