#include <bits/stdc++.h>
using namespace std;
int main() {
  int x1, x2, y1, y2;
  scanf("%d%d%d%d", &x1, &y1, &x2, &y2);
  if (x1 > x2) {
    swap(x1, x2);
  }
  if (y1 > y2) {
    swap(y1, y2);
  }
  int xj, xo, yj, yo;
  int xs = x2 - x1 + 1;
  int ys = y2 - y1 + 1;
  xj = xo = xs / 2;
  yj = yo = ys / 2;
  if (xs % 2) {
    if (x1 % 2) {
      xj++;
    } else {
      xo++;
    }
  }
  if (ys % 2) {
    if (y1 % 2) {
      yj++;
    } else {
      yo++;
    }
  }
  if ((x1 + y1) % 2) {
    swap(xj, xo);
  }
  unsigned long long int ans = 0;
  ans = (unsigned long long int)xj * (unsigned long long int)yj +
        (unsigned long long int)xo * (unsigned long long int)yo;
  printf("%I64u\n", ans);
  return 0;
}
