#include <bits/stdc++.h>
using namespace std;
long long MOD = 1000000007;
long long qpow(long long a, long long b) {
  long long ans = 1;
  while (b) {
    if (b & 1) ans = (ans * a) % MOD;
    a = (a * a) % MOD;
    b >>= 1;
  }
  return ans;
}
long long inverse(long long a) { return qpow(a, MOD - 2) % MOD; }
int gcd(int a, int b) { return b == 0 ? a : gcd(b, a % b); }
int extend_gcd(int a, int b, int &x, int &y) {
  if (b == 0) {
    x = 1;
    y = 0;
    return a;
  } else {
    int r = extend_gcd(b, a % b, y, x);
    y -= x * (a / b);
    return r;
  }
}
int main() {
  long long x1, x2, y1, y2, dy, ans;
  cin >> x1 >> y1 >> x2 >> y2;
  ans = ((y2 - y1) / 2) * ((x2 - x1) / 2) +
        ((x2 - x1) / 2 + 1) * ((y2 - y1) / 2 + 1);
  cout << ans << endl;
  return 0;
}
