#include <bits/stdc++.h>
using namespace std;

int x, y, p, q;

int visited[101][101][2];

/*void dfs(int s, int w, bool john, int moves) {
	if (moves >= dp[s][w][john]) return;
	dp[s][w][john] = moves;
	if (john && s == x) {
		ans = min(ans, moves);
		return;
	}
	for (int s1 = 0; s1 <= min(s, p); s1++) {
		for (int w1 = 0; w1 <= min(w, p - s1); w1++) {
			if ((w - w1) - (s - s1) <= q) {
				dfs((x - s) + s1, (y - w) + w1, !john, moves + 1);
			}
		}
	}
}*/

int main() {
	int ans = 0;
	bool go = true;

	cin >> x >> y >> p >> q;

	queue<pair<int, pair<int, int> > > bfs[11000];

	bfs[0].push(make_pair(0, make_pair(x, y)));

	while (go) {
		if (bfs[ans].empty()) {
			cout << -1 << endl;
			return 0;
		}
		while (!bfs[ans].empty()) {
			int john = bfs[ans].front().first, s = bfs[ans].front().second.first, w = bfs[ans].front().second.second;
			if (john && s == x) {
				cout << ans << endl;
				return 0;
			}
			for (int s1 = 0; s1 <= min(s, p); s1++) {
				for (int w1 = 0; w1 <= min(w, p - s1); w1++) {
					if (((w - w1) - (s - s1) <= q) || (s == s1)) {
						if (!visited[(x - s) + s1][(y - w) + w1][!john]) {
							bfs[ans + 1].push(make_pair(!john, make_pair((x - s) + s1, (y - w) + w1)));
							//cout << ans << " " << !john << " " << (x - s) + s1 << " " << (y - w) + w1 << endl;
							visited[(x - s) + s1][(y - w) + w1][!john] = 1;
						}
					}
				}
			}
			bfs[ans].pop();
		}
		ans++;
	}
}