#include <bits/stdc++.h>

using i64 = long long;

const int INF = 1E9;

struct Node {
    int cnt, sp, wv;
    bool operator < (const Node &u) const {
        return cnt > u.cnt;
    }
};

void solve() {
    int x, y, p, q;
    std::cin >> x >> y >> p >> q;
    std::priority_queue<Node> pq;
    std::vector vis(400, std::vector<int>(400));
    int ans = INF;
    pq.push({0, 0, 0});
    while (!pq.empty()) {
        auto [c, a, b] = pq.top();
        pq.pop();

        // std::cout << c << " " << a << " " << b << "\n";

        assert(b <= y); 
        vis[a][b] = 1;
        
        if (a == x) {
            ans = std::min(ans, c);
            continue;
        }
        if (c % 2 == 0) {
            for (int i = 0; i <= x - a && i <= p; ++i) {
                if (y - b - p + i > x - a - i + q && a + i != x) {
                    continue;
                }
                if (vis[a + i][std::min(y - b, b + p - i)]) continue;
                pq.push({c + 1, a + i, std::min(y - b, b + p - i)});
            }
        } else {
            for (int i = 0; i <= b && i <= p; ++i) {
                if (b - i > a + q && a != 0) continue;
                pq.push({c + 1, a, b - i});
            }
        }
    }
    if (ans == INF) ans = -1;
    std::cout << ans << "\n";
}

int main() {
    std::ios::sync_with_stdio(false);
    std::cin.tie(nullptr);
    int T = 1;
    // std::cin >> T;
    while(T--) {
        solve();
    }
}