#include <bits/stdc++.h>
using namespace std;
int a, b, x, y;
long long row, col, ans, aa, bb;
long long MAX(long long a, long long b) {
  if (a > b)
    return a;
  else
    return b;
}
long long solve() {
  col = (long long)(x - a + 1);
  if ((a + b) % 2 != 0 && (a + y) % 2 != 0) {
    row = (long long)((y - b) / 2);
    ans += (col / 2ll + 1ll) * row;
  } else {
    row = (long long)((y - b) / 2 + 1);
    ans += (col / 2ll + 1ll) * row;
  }
  x--;
  if ((x + b) % 2 != 0 && (x + y) % 2 != 0) {
    row = (long long)((y - b) / 2);
    ans += (col / 2ll) * row;
  } else {
    row = (long long)((y - b) / 2 + 1);
    ans += (col / 2ll) * row;
  }
  x++;
  return ans;
}
int main() {
  scanf("%d %d %d %d", &a, &b, &x, &y);
  ans = 0ll;
  aa = solve();
  b++;
  y++;
  ans = 0ll;
  bb = solve();
  printf("%I64d\n", MAX(aa, bb));
  return 0;
}
