#include <iostream>
#include <cstdio>
#include <cstring>
#include <algorithm>
#include <queue>

using namespace std;

const int N = 110;

typedef pair<int, int> PII;

struct Node {
	int a, b, c;
};

int f[N][N][5];
bool st[N][N][5];
int x, y, p, q;

void bfs()
{
	memset(f, 0x3f, sizeof f);
	f[x][y][0] = 0;
	queue<Node> qq;
	qq.push({ x, y, 0 });
	st[x][y][0] = true;

	while (!qq.empty())
	{
		auto t = qq.front();
		qq.pop();
		int a = t.a, b = t.b, c = t.c;
		int aa = x - a, bb = y - b;
		
		for (int i = 0; i <= a && i <= p; i++)
			for (int j = 0; j <= b && j + i <= p; j++)
			{
				if ((b - j <= a - i + q || a - i == 0) && !st[aa + i][bb + j][!c]) {
					st[aa + i][bb + j][!c] = true;
					f[aa + i][bb + j][!c] = min(f[aa + i][bb + j][!c], f[a][b][c] + 1);
					qq.push({ aa + i, bb + j, !c });
				}
			}
	}

}

int main()
{
	//int xxx = 0;
	//cout << !xxx << endl;
	cin >> x >> y >> p >> q;
	bfs();
	int ans = 0x3f3f3f3f;


	/*for (int k = 0; k <= 1; k++)
	{
		for (int i = 0; i <= x; i++)
			for (int j = 0; j <= y; j++)
				cout << i << " " << j << " " << f[i][j][k] << endl;
		cout << endl;
	}*/

	for (int i = 0; i <= y; i++)
	{
		//cout << f[0][i] << endl;
		ans = min(ans, f[x][i][1]);
	}
	if (ans == 0x3f3f3f3f) {
		cout << -1 << endl;
		return 0;
	}
	cout << ans << endl;
	return 0;
}