#include <bits/stdc++.h>
using namespace std;
#define rep(i,a,b) for(int i = a; i < (b); ++i)
#define all(x) begin(x),end(x)
#define sz(x) (int) (x).size();
typedef long long ll;
typedef pair<int,int> pii;
typedef vector<int> vi;

const int kN = 101;
int dis[2][kN][kN];

int main() {
	cin.tie(0) -> sync_with_stdio(0);
	cin.exceptions(cin.failbit);

	int x, y, p, q;
	cin >> x >> y >> p >> q; 

	rep (i, 0, x + 1) rep (j, 0, y + 1) dis[0][i][j] = -1;
	rep (i, 0, x + 1) rep (j, 0, y + 1) dis[1][i][j] = -1;

	dis[0][x][y] = 0;
	
	queue<tuple<int, int, int>> que;
	que.emplace(0, x, y);

	while (not que.empty()) {
		auto [s, i, j] = que.front();
		que.pop();
		if (i == 0) {
			cout << dis[s][i][j] << '\n';
			return 0;
		}
		if (s == 0) {
			rep (ni, 0, i + 1) rep (nj, 0, j + 1) {
				if ((nj > ni + q and ni > 0) or (i - ni) + (j - nj) > p) continue;
				if (dis[1][ni][nj] != -1) continue;
				dis[1][ni][nj] = dis[0][i][j] + 1;
				que.emplace(1, ni, nj);
			}
		} else {
			rep (ni, 0, x - i + 1) rep (nj, 0, y - j + 1) {
				if ((nj > ni + q and ni > 0) or ((x - i) - ni) + ((y - j) - nj) > p) continue;
				if (dis[0][x - ni][y - nj] != -1) continue;
				dis[0][x - ni][y - nj] = dis[1][i][j] + 1;
				que.emplace(0, x - ni, y - nj);
			}
		}
	}

	cout << -1 << '\n';
}