#include <bits/stdc++.h>

using i64 = long long;

const int N = 110;
int X, Y, p, q;
int dp[N][N][2];

bool check(int x, int y, int op) {
    if (!op) {
        return X - x && Y - y > X - x + q;
    }
    return x && y > x + q;
}

int main() {
    std::ios::sync_with_stdio(false);
    std::cin.tie(nullptr);

    std::cin >> X >> Y >> p >> q;
    for (int i = 0; i <= X; ++i) {
        for (int j = 0; j <= Y; ++j) {
            dp[i][j][0] = dp[i][j][1] = -1;
        }
    }

    dp[X][Y][0] = 0;
    std::queue<std::array<int, 3>> Q;
    Q.push({X, Y, 0});
    int ans = 1e9;
    while (!Q.empty()) {
        auto t = Q.front(); Q.pop();
        int x = t[0], y = t[1], op = t[2];
        
        int limx = op ? X - x : x, limy = op ? Y - y : y;
        for (int i = 0; i <= limx; ++i) {
            for (int j = 0; j <= limy; ++j) {
                int nx = op ? x + i : x - i, ny = op ? y + j : y - j;
                if (!check(nx, ny, op ^ 1) && dp[nx][ny][op ^ 1] == -1 && i + j <= p) {
                    dp[nx][ny][op ^ 1] = dp[x][y][op] + 1;
                    Q.push({nx, ny, op ^ 1});
                }
            }
        }
    }
    for (int i = 0; i <= Y; ++i) {
        if (dp[0][i][0] != -1) {
            ans = std::min(ans, dp[0][i][0]);
        }
    }
    if (ans == 1e9) {
        std::cout << "-1\n";
    } else {
        std::cout << ans - 1 << "\n";
    }
    return 0;
}