#include <bits/stdc++.h>
using namespace std;
int m, n, x, y;
int ABS(int x) { return (x < 0) ? (-x) : (x); }
int main() {
  scanf("%d%d%d%d", &m, &x, &n, &y);
  if (m > n) {
    swap(m, n);
    swap(x, y);
  }
  int d = ABS(x - y);
  long long ans = 1 + m + n;
  for (int i = 1; i <= m; i++) {
    int b = i + d - 1, a = ABS(d - i) + 1;
    a = max(a, 1);
    b = min(b, n);
    if (a <= b) ans += (b - a + 1) * 2 - 1;
    if (i > d) ans++;
  }
  cout << ans << endl;
  return 0;
}
