#include <bits/stdc++.h>
using i64 = long long;
const int INF = 1E4;

void solve() {
    int x, y, p, q;
    std::cin >> x >> y >> p >> q;
    std::vector dp(2, std::vector(x + 1, std::vector<int>(y + 1, INF)));
    dp[0][0][0] = dp[1][x][y] = 0;
    int opt = 0;
    for(int t = 0; t <= x * 2; ++t, opt ^= 1) {
        for(int i = 0; i <= p; ++i) { 
            for(int j = 0; j <= p - i; ++j) { 
                for(int a = i; a <= x; ++a) { 
                    for(int b = (a == x ? j : std::max(j, y - x + a - q)); b <= y; ++b) {
                        dp[opt][a][b] = std::min(dp[opt][a][b], dp[opt ^ 1][x - a + i][y - b + j] + 1); 
                    }
                }
            }
        }
    }   
    i64 sum = 0; 
    int mn = INF;
    for(int i = 0; i <= y; ++i) {
        mn = std::min(mn, dp[0][x][i]);
    }
    std::cout << (mn == INF ? -1 : mn) << '\n';
}

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