-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;
long long mod(long long a, long long b) {
  if (a % b < 0) {
    return a % b + b;
  }
  return a % b;
}
long long mod_exp(long long a, long long b, long long c) {
  long long res = 1;
  a = a % c;
  while (b > 0) {
    if (b % 2 == 1) {
      res = (res * a) % c;
    }
    b /= 2;
    a = (a * a) % c;
  }
  return res;
}
long long mymod(long long a, long long b) { return ((a % b) + b) % b; }
bool prime[200001];
void SieveOfEratosthenes() {
  memset(prime, true, sizeof(prime));
  prime[1] = false;
  for (int p = 2; p * p <= 200000; p++) {
    if (prime[p] == true) {
      for (int i = p * p; i <= 200000; i += p) prime[i] = false;
    }
  }
  return;
}
long long powe[200005];
void power() {
  powe[0] = 1;
  for (long long i = 1; i < 200005; i++) {
    powe[i] = mymod(2 * powe[i - 1], 1000000007);
  }
}
long long gcdExtended(long long, long long, long long *, long long *);
long long modInverse(long long a, long long m) {
  long long x, y;
  long long g = gcdExtended(a, m, &x, &y);
  long long res = (x % m + m) % m;
  return res;
}
long long gcdExtended(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 gcd = gcdExtended(b % a, a, &x1, &y1);
  *x = y1 - (b / a) * x1;
  *y = x1;
  return gcd;
}
long long facto[200005];
void first() {
  facto[0] = 1;
  for (long long i = 1; i < 200005; i++) {
    facto[i] = mymod((i * facto[i - 1]), 1000000007);
  }
}
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  long long x1, y1, x2, y2;
  cin >> x1 >> y1 >> x2 >> y2;
  long long evenx, oddx, eveny, oddy;
  long long dx = x2 - x1;
  long long dy = y2 - y1;
  if (x1 % 2 == x2 % 2) {
    if (x1 & 1) {
      oddx = (dx) / 2 + 1;
      evenx = (dx) / 2;
    } else {
      oddx = (dx) / 2;
      evenx = (oddx) + 1;
    }
  } else {
    oddx = evenx = (x2 - x1 + 1) / 2;
  }
  if (y1 % 2 == y2 % 2) {
    if (y1 & 1) {
      oddy = (dy) / 2 + 1;
      eveny = oddy - 1;
    } else {
      oddy = (dy) / 2;
      eveny = oddy + 1;
    }
  } else {
    oddy = eveny = (y2 - y1 + 1) / 2;
  }
  cout << oddy * oddx + eveny * evenx << "\n";
  return 0;
}
