#include<bits/stdc++.h>

using namespace std;

const int X = 105;

bool vis[X][X][2];
int dist[X][X][2];


int x, y, p, q;

bool feasible(int a, int b, int s) {
    if(a < 0 || a > x || b < 0 || b > y) return false;
    if(s == 0) {
        return y - b <= x - a + q || x - a == 0;
    } else {
        return b <= a + q || a == 0;
    }
}

int main() {
    cin.tie(0);
    ios::sync_with_stdio(0);
    cin >> x >> y >> p >> q;

    queue<tuple<int, int, int>> qu;
    qu.push({x, y, 0});
    vis[x][y][0] = 1;
    while(!qu.empty()) {
        int a, b, s;
        tie(a, b, s) = qu.front();
        qu.pop();
        for(int m = 0; m <= p; m++) {
            for(int n = 0; n <= p; n++) {
                if(m+n > p) {
                    continue;
                }
                if(s == 0) {
                    if(feasible(a-m, b-n, 1) && !vis[a-m][b-n][1]) {
                        qu.push({a-m, b-n, 1});
                        vis[a-m][b-n][1] = 1;
                        dist[a-m][b-n][1] = dist[a][b][s] + 1;
                    }
                } else {
                    if(feasible(a+m, b+n, 0) && !vis[a+m][b+n][0]) {
                        qu.push({a+m, b+n, 0});
                        vis[a+m][b+n][0] = 1;
                        dist[a+m][b+n][0] = dist[a][b][s] + 1;
                    }
                }
            }
        }
    }

    int ans = 1e9;
    for(int w = 0; w <= y; w++) {
        if(vis[0][w][1]) {
            ans = min(ans, dist[0][w][1]);
        }
    }
    if(ans < (int) 1e9) {
        cout << ans << endl;
    } else {
        cout << -1 << endl;
    }
    // cout << ans << endl;
}