#include<bits/stdc++.h>
using namespace std;
const int inf = 0x3f3f3f3f;
int x, y, p, q, dp[105][105][2];
int Check(int a, int b, int c) {
	if(c == 0)
		return x - a && y - b > x - a + q;
	return a && b > a + q;
}
struct Node {
	int a, b, c;
	Node(int a = 0, int b = 0, int c = 0) : a(a), b(b), c(c) {}
};
queue<Node>que;
int main() {
	memset(dp, -1, sizeof dp);
	scanf("%d%d%d%d", &x, &y, &p, &q);
	dp[x][y][0] = 0;
	que.push(Node(x, y, 0));
	while(!que.empty()) {
		Node now = que.front();
		int a = now.a, b = now.b, c = now.c;
		que.pop();
		if(!a)
			return 0 * printf("%d\n", dp[a][b][c]);
		int limi = c ? x - a : a, limj = c ? y - b : b;
		for(int i = 0; i <= limi; ++ i) {
			for(int j = 0; j <= limj; ++ j) {
				if(i + j > p)
					continue;
				int tmpi = c ? a + i : a - i, tmpj = c ? b + j : b - j;
				if(!Check(tmpi, tmpj, c ^ 1) && dp[tmpi][tmpj][c ^ 1] == -1) {
					dp[tmpi][tmpj][c ^ 1] = dp[a][b][c] + 1;
					que.push(Node(tmpi, tmpj, c ^ 1));
				}
			}
		}
	}
	puts("-1");
}