#include <bits/stdc++.h>

using namespace std;

#define iter(v) v.begin(), v.end()
#define SZ(v) int(v.size())
#define pb emplace_back
#define ff first
#define ss second

using ll = long long;
using pii = pair<int, int>;
using pll = pair<ll, ll>;

#ifdef zisk
void debug(){cerr << "\n";}
template<class T, class ... U>
void debug(T a, U ... b){cerr << a << " ", debug(b...);}
template<class T> void pary(T l, T r){
	while (l != r) cerr << *l << " ", l++;
	cerr << "\n";
}
#else
#define debug(...) void()
#define pary(...) void()
#endif

template<class A, class B>
ostream &operator<<(ostream& o, pair<A, B> p) {
	return o << '(' << p.ff << ',' << p.ss << ')';
}

struct item {
	int x, y, side;
};

int main() {
	ios_base::sync_with_stdio(false);
	cin.tie(0);

	int X, Y, P, Q;
	cin >> X >> Y >> P >> Q;

	vector dis(X + 1, vector(Y + 1, vector<int>(2, -1)));
	// x, y: 0
	auto check = [&](int x, int y, int side){
		if (side == 0) x = X - x, y = Y - y;
		if (x == 0) return true;
		if (y > x + Q) return false;
		return true;
	};

	queue<item> q;
	q.push(item({X, Y, 0}));
	dis[X][Y][0] = 0;
	
	auto relax = [&](int x, int y, int side, int d) {
		//debug("relax", x, y, side, d);
		if (!check(x, y, side)) return;
		if (dis[x][y][side] != -1) return;
		dis[x][y][side] = d;
		q.push(item({x, y, side}));
	};

	while (!q.empty()) {
		auto [x, y, side] = q.front();
		q.pop();
		//debug("test", x, y, side, dis[x][y][side]);

		if (side == 0) {
			for (int cx = 0; cx <= x; cx++)
				for (int cy = 0; cy <= y && cx + cy <= P; cy++)
					relax(x - cx, y - cy, !side, dis[x][y][side] + 1);
		}
		else {
			for (int cx = 0; cx <= X - x; cx++)
				for (int cy = 0; cy <= Y - y && cx + cy <= P; cy++)
					relax(x + cx, y + cy, !side, dis[x][y][side] + 1);
		}
	}

	int ans = -1;
	for (int y = 0; y <= Y; y++) {
		if (dis[0][y][1] == -1) continue;
		if (ans == -1 || ans > dis[0][y][1])
			ans = dis[0][y][1];
	}
	cout << ans << "\n";

}