-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 int DEBUG = 0;
namespace defs {
int read() {
  int aa, bb = 1, ch;
  while (ch = getchar(), (ch < '0' || ch > '9') && ch != '-')
    ;
  ch == '-' ? aa = 0, bb = -1 : aa = ch - '0';
  while (ch = getchar(), ch >= '0' && ch <= '9') aa = aa * 10 + ch - '0';
  return aa * bb;
}
const unsigned int rand_x = 1401010315;
const unsigned int rand_y = 1000000019;
unsigned int rand_cur;
unsigned int rand_int() { return rand_cur = rand_cur * rand_x + rand_y; }
const int inf = 1071026353;
const int mod = 1000000007;
const int modx = 998244353;
}  // namespace defs
namespace lightningwork {
using namespace defs;
void work() {
  long long x1 = read();
  long long y1 = read();
  long long x2 = read();
  long long y2 = read();
  long long y = y2 - y1;
  long long x = x2 - x1;
  y1 = (y1 % 2 + 2) % 2;
  y2 = y1 + y;
  x1 = (x1 % 2 + 2) % 2;
  x2 = x1 + x;
  long long a = y2 / 2;
  long long b = y2 - a;
  long long c = x2 / 2;
  long long d = x2 - c;
  long long ans = b * d + a * c;
  if (x1 == 0) ans += (y2 + 1) / 2;
  if (y1 == 0) ans += x2 / 2;
  printf(
      "%lld"
      "\n",
      ans);
}
}  // namespace lightningwork
int main() {
  if (0)
    freopen(
        "work"
        ".in",
        "r", stdin),
        freopen(
            "work"
            ".out",
            "w", stdout);
  lightningwork::work();
  if (0) fclose(stdin), fclose(stdout);
  return 0;
}
