//
#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
#define debug(...) fprintf (stderr, __VA_ARGS__)

#define lep(i, l, r) for (int i = (l), i##_end = (r); i <= i##_end; ++ i)
#define rep(i, r, l) for (int i = (r), i##_end = (l); i >= i##_end; -- i)

char _c; bool _f; template <class T> inline void IN (T & x) {
	_f = 0, x = 0; while (_c = getchar (), ! isdigit (_c)) if (_c == '-') _f = 1;
	while (isdigit (_c)) x = x * 10 + _c - '0', _c = getchar (); if (_f == 1) x = -x;
}

template <class T> inline void chkmin (T & x, T y) { if (x > y) x = y; }
template <class T> inline void chkmax (T & x, T y) { if (x < y) x = y; }

const int inf = 1e9 + 7;

int x, y, p, q;

inline bool check (int a, int b) {
	return ! a || (b <= a + q);
}

const int NS = 3e4 + 5;

int dist[NS];
inline int id (int i, int j, int o) {
	return i * (y + 1) + j + o * (x + 1) * (y + 1);
}

signed main () {
	IN (x), IN (y), IN (p), IN (q);
	queue <tuple <int, int, int> > Q;
	memset (dist, -1, sizeof dist);
	Q.emplace ( x, y, 0 ), dist[id (x, y, 0)] = 0;
	while (! Q.empty ()) {
		auto [i, j, o] = Q.front (); Q.pop ();
		int u = id (i, j, o);
		if (i == 0 && o == 1) {
			return printf ("%d\n", dist[u]), 0;
		}
		if (! o) {
			lep (a, 0, i) lep (b, 0, j) if (a + b <= p) {
				if (! check (i - a, j - b)) continue ;
				int v = id (i - a, j - b, 1);
				if (! ~ dist[v]) dist[v] = dist[u] + 1, Q.emplace ( i - a, j - b, 1 );
			}
		} else {
			lep (a, 0, x - i) lep (b, 0, y - j) if (a + b <= p) {
				if (! check (x - i - a, y - j - b)) continue ;
				int v = id (i + a, j + b, 0);
				if (! ~ dist[v]) dist[v] = dist[u] + 1, Q.emplace ( i + a, j + b, 0 );
			}
		}
	}
	return puts ("-1"), 0;
}