#include <bits/stdc++.h>
using namespace std;
inline int in() {
  int res = 0;
  char c;
  int f = 1;
  while ((c = getchar()) < '0' || c > '9')
    if (c == '-') f = -1;
  while (c >= '0' && c <= '9') res = res * 10 + c - '0', c = getchar();
  return res * f;
}
const int N = 100010;
int main() {
  long long x1 = in(), y1 = in(), x2 = in() - x1 + 1, y2 = in() - y1 + 1;
  x1 = x2, y1 = y2;
  long long ans = 0;
  if (x1 & 1) {
    long long t = x1 / 2 + 1;
    ans += (y1 + 1) / 2 * t;
    t--;
    ans += y1 / 2 * t;
    cout << ans;
  } else {
    long long t = x1 / 2;
    ans += (y1 + 1) / 2 * t;
    ans += y1 / 2 * t;
    cout << ans;
  }
  return 0;
}
