#include <bits/stdc++.h>
using namespace std;
struct Node {
	int type, x, y;
};

queue<Node> Q;
int dp[109][109][2];

int main() {
	//freopen("E.in", "r", stdin);
	for (int i = 0; i < 109; i++) for (int j = 0; j < 109; j++) for (int k = 0; k < 2; k++) dp[i][j][k] = INT_MAX;
	int x, y, p, q;
	cin >> x >> y >> p >> q;
	Q.push({0, x, y});
	dp[x][y][0] = 0;
	while (Q.size()) {
		auto cur = Q.front(); Q.pop();
		int type = cur.type, X = cur.x, Y = cur.y;
		int cx = (type == 0 ? X : x - X);
		int cy = (type == 0 ? Y : y - Y);
		for (int px = 0; px <= cx; px++) {
			for (int py = 0; py <= cy && py + px <= p; py++) {
				if (px < cx && cy - py > cx - px + q) continue;
				int nx = (type == 0 ? X - px : X + px);
				int ny = (type == 0 ? Y - py : Y + py);
				if (dp[X][Y][type] + 1 < dp[nx][ny][type ^ 1]) {
					dp[nx][ny][type ^ 1] = dp[X][Y][type] + 1;
					Q.push({type ^ 1, nx, ny});	
				}
			}
		}
	}
	
	int ans = INT_MAX;
	for (int i = 0; i <= y; i++) {
		ans = min(ans, min(dp[0][i][0], dp[0][i][1]));	
	}
	
	if (ans == INT_MAX) cout << -1;
	else cout << ans;
	return 0;
}