-157778763 218978790 976692563 591093088
 -1 -4 1 4
 -999999999 -1000000000 -1 0
 1000000000 1000000000 1000000000 1000000000
 -2 -3 -2 1
 -1000000000 -999999999 1000000000 999999999
 0 -1 0 1
 -999999999 -999999999 999999999 999999999
 -946749893 -687257665 -539044455 -443568671
 -26644507 -867720841 975594569 264730225
 309857887 -687373066 663986893 403321752
 -2 -2 -2 0
 0 -2 0 2
 0 -1 2 1
 -2 -2 2 2
 -999999999 -1000000000 999999999 1000000000
 -330513944 -970064382 500608496 369852884
 0 -3 0 3
 0 0 2 2
 -946749893 -687257666 -539044455 -443568670
 -330513944 -970064383 500608496 369852885
 -1000000000 -1000000000 1000000000 1000000000
 0 -1 2 3
 -999999999 -999999999 -1 -1
 309857887 -687373065 663986893 403321751
 -482406510 -512306895 412844236 -168036049
 0 0 2 0
 0 0 0 0
 -471257905 -685885154 782342299 909511044
 -482406510 -512306894 412844236 -168036050
 -157778763 218978791 976692563 591093087
 -411495869 33834653 -234317741 925065545
 -2 -3 2 3
 -411495869 33834652 -234317741 925065546
 -26644507 -867720842 975594569 264730226
 -537640548 -254017710 62355638 588691834
 -471257905 -685885153 782342299 909511043
 -1 -3 1 3
 1000000000 999999999 1000000000 999999999
 1 0 5 6
 -537640548 -254017711 62355638 588691835

211076501291102387 14 500000000000000000 1 3 2000000000000000000 2 1999999998000000001 49676664342971903 567493356068872580 193123336596257367 2 3 5 13 2000000000000000000 556817654843544374 4 5 49676664750677342 556817655674666815 2000000002000000001 8 499999999000000001 193123336242128360 154104366473536355 2 1 999994501061310398 154104365578285608 211076500156631060 78953311064369599 18 78953311241547728 567493357071111657 252811256874252458 999994499807710193 11 1 18 252811257474248645
#include <bits/stdc++.h>
using namespace std;
const long double eps = 0.000000001;
const double pi = 3.141592653589;
const long long mod = 1000000007;
long long gcd(long long a, long long b) {
  if (a > b) swap(a, b);
  while (a != 0 && b != 0) {
    b %= a;
    swap(a, b);
  }
  return a + b;
}
long long poww(long long v, long long p) {
  if (p == 0) return 1;
  if (p & 1) {
    return (poww(v, p - 1) * v) % mod;
  } else {
    long long t = poww(v, p / 2);
    return (t * t) % mod;
  }
}
void calc_primes(int n, vector<bool> &primes) {
  primes.resize(n, true);
  primes[1] = false;
  for (int i = 2; i * i < n; ++i) {
    if (!primes[i]) continue;
    for (int j = i * i; j < n; j += i) {
      primes[j] = false;
    }
  }
}
long long ext_gcd(long long a, long long b, long long &x, long long &y) {
  if (a == 0) {
    x = 0;
    y = 1;
    return b;
  }
  long long x1, y1;
  long long d = ext_gcd(b % a, a, x1, y1);
  x = y1 - (b / a) * x1;
  y = x1;
  return d;
}
bool vect_comp(pair<int, int> a, pair<int, int> b) {
  return (a.first * b.second - a.second * b.first > 0);
}
bool get_bit(int &v, char &num) { return ((v >> num) & 1); }
void set_bit(int &v, char &num) { v = v | (1 << num); }
int count_bit(int v) {
  int cnt = 0;
  while (v > 0) {
    cnt += v & 1;
    v = v >> 1;
  }
  return cnt;
}
long long n, m, p, g, w;
long long t;
long long res;
long long x1, yy1, x2, y2;
int arr[211];
int main() {
  cin >> x1 >> yy1 >> x2 >> y2;
  long long res = (x2 - x1) / 2 * (y2 - yy1 + 1);
  res += (y2 - yy1) / 2;
  res += x1 % 2;
  cout << res;
  return 0;
  return 0;
}
