#include <bits/stdc++.h>
using namespace std;

#define PB emplace_back
#define int long long
#define rep(i, a, b) for(int i = (a); i <= (b); ++i)
#define per(i, a, b) for(int i = (a); i >= (b); --i)
#define siz(a) ((int) a.size())

int a, b, c, d, f[105][105][2];
struct pii {int x, y, z;};

signed main() {
	// freopen(".in", "r", stdin);
	// freopen(".out", "w", stdout);
	ios::sync_with_stdio(0);
	cin.tie(0), cout.tie(0);
	cin >> a >> b >> c >> d;
	queue<pii> qu;
	memset(f, 0x3f, sizeof(f));
	f[a][b][0] = 0;
	qu.push(pii {a, b, 0});
	int ans = 1e9;
	while(siz(qu)) {
		auto [x, y, z] = qu.front();
		qu.pop();
		// cout << x << ' ' << y << ' ' << z << endl;
		if(!z && x <= c) ans = min(ans, f[x][y][z] + 1);
		rep(i, 0, x) rep(j, 0, y) if(i + j <= c) {
			int p = x - i, q = y - j;
			if(q > p + d && p) continue;
			if(f[a - p][b - q][z ^ 1] <= f[x][y][z] + 1) continue;
			f[a - p][b - q][z ^ 1] = f[x][y][z] + 1;
			qu.push(pii {a - p, b - q, z ^ 1});
		}
	}
	ans == 1e9 ? cout << -1 : cout << ans;
	return 0;
}