#include <bits/stdc++.h>

#define ll long long

constexpr int N = 101;

int x, y, p, q, dp[N][N][2];// 0, 1
int cnt = 0;
void bfs(int ii, int jj, int tt){
	std::queue<std::array<int, 3>> qp;
	qp.push({ii, jj, tt});
	while (!qp.empty()){
		auto [i, j, t] = qp.front();
		qp.pop();
		for (int u = std::max(0, i - p); u <= i; u++){
			for (int v = std::max(0, j - p); v <= j; v++){
				int change = std::abs(i - u) + std::abs(j - v);
				if (change > p) {
					continue;
				}
				int r = x - u, c = y - v;
				if (u + q < v && u != 0) {
					continue;
				}
				if (dp[r][c][t ^ 1] > dp[i][j][t] + 1){
					dp[r][c][t ^ 1] = dp[i][j][t] + 1;
					qp.push({r, c, t ^ 1});
				}
			}
		}
	}
}

int main(){
	std::ios::sync_with_stdio(false);
	std::cin.tie(nullptr);

	std::cin >> x >> y >> p >> q;
	memset(dp, 127, sizeof(dp));
	dp[x][y][0] = 0;
	bfs(x, y, 0);
	int ans = 1E9;
	for (int i = 0;i <= y; i++){
		ans = std::min(ans, dp[x][i][1]);
	}
	if (ans == 1e9){
		std::cout << "-1\n";
	}else{
		std::cout << ans << '\n';
	}

	return 0;
}