#include <bits/stdc++.h>
using namespace std;
int x, y, p, q; map<vector<int>, bool> vis; map<vector<int>, int> dist;


int main() {
    cin >> x >> y >> p >> q;
    queue<vector<int>> qu; qu.push({x, y, 0}); vis[{x, y, 0}] = true; dist[{x, y, 0}] = 0;

    while (!qu.empty()) {
        vector<int> f = qu.front(); qu.pop();
        int a = f[0], b = f[1], c = f[2];

        if (a == x && c == 1) {
            cout << dist[{a, b, c}]; return 0;
        }

        for (int j = 0; j <= p; ++j) {
            for (int i = 0; i <= j; ++i) {
                int takingsheep = i, takingwolves = j - i;
                if (takingwolves < 0) continue;

                int leftsheep = a - takingsheep, leftwolves = b - takingwolves;
                if (leftsheep < 0 || leftwolves < 0) continue;

                int mysidesheep = takingsheep + x - a, mysidewolves = takingwolves + y - b;
                if (mysidesheep < 0 || mysidewolves < 0) continue;
                
                if ((!leftsheep || leftsheep + q >= leftwolves) && !vis[{mysidesheep, mysidewolves, !c}]) {
                    vis[{mysidesheep, mysidewolves, !c}] = true;
                    dist[{mysidesheep, mysidewolves, !c}] = dist[{a, b, c}] + 1;
                    qu.push({mysidesheep, mysidewolves, !c});
                }
            }
        }
    }
    cout << -1;
    return 0;
}