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

using namespace std;

int main()
{
	int x1, y1, p, q;
	cin >> x1 >> y1 >> p >> q;
	int x2 = 0, y2 = 0;
	int xx = 0, yy = 0;
	int t = true;
	int ans = 0;
	if (y1 - p - x1 == q) {
		cout << (x1 + p - 1) / p * 2 + 1 << endl;
		return 0;
	}
	while (1)
	{
		//cout << x1 << " " << y1 << endl;
		xx = 0, yy = 0;
		if (y1 - p - x1 >= q) {
			ans = -1;
			break;
		}
		for (int i = 0; i <= p; i++)
			if (y1 - i - (x1 - (p - i)) <= q) {
				yy = i, xx = p - i;
				y1 -= i, x1 -= (p - i);
			}
		if (x1 <= 0) {
			ans++;
			break;
		}
		x2 += xx;
		if (x2 + p <= y2 + yy) {
			int t = x2 + p - y2;
			y2 = x2 + p;
			yy = yy - t;
		}
		else {
			y2 += yy;
			yy = 0;
		}
		y1 += yy;
		ans += 2;
		
	}
	cout << ans << endl;
}