#include <bits/stdc++.h>

#define x first
#define y second
using namespace std;
using PII = pair<int, int>;
#define int long long
#define endl "\n"

const int N = 201;

int dist[N][N][2];
struct Node{
    int x, y, op;
};

void solve() {

    int x,y,p,q;cin >> x >> y >> p >> q;

    queue<Node> que;
    que.push({x, y, 0});
    memset(dist, -1, sizeof dist);
    dist[x][y][0] = 0;
    while(que.size()){
        auto [x1, y1, op] = que.front();
        // cout << x1 << " " << y1 << " " << (x - x1) << " " << (y - y1) << " " << op << " " << dist[x1][y1][op] << "\n";

        que.pop();
        if(op == 0) {
            if((y - y1) - (x - x1) > q && (x - x1) != 0) continue;
        }else {
            if(y1 - x1 > q && x1 != 0) continue;
        }

        if(x1 == 0){
            // cout << x1 << " " << y1 << "\n";
            cout << dist[x1][y1][op] << "\n";
            return;
        }
        int x2 = x - x1, y2 = y - y1;
        if(op % 2 == 0){

            for(int i = 0; i <= min(x1, p); i ++){
                // cout << i + (p - i) << endl;
                int lx = x1 - i, ly = max(0LL, y1 - (p - i));
                // cout << x1 << "  " << y1 << " " << i << endl;
                if(ly - lx > q && lx != 0) continue;
                if(dist[lx][ly][op ^ 1] != -1) continue;


                dist[lx][ly][op ^ 1] = dist[x1][y1][op] + 1;
                que.push({lx, ly, op ^ 1});
            }
        }
        else{
            for(int i = 0; i <= min(x2, p); i ++){
                for(int j = 0; j <= min(y2, p - i); j ++){
                    // cout << i + j << endl;
                    int lx = (x1 + i), ly = (y1 + j);

                    if((y - ly) - (x - lx) > q && (x - lx) != 0) continue;
                    if(dist[lx][ly][op ^ 1] != -1) continue;
                    dist[lx][ly][op ^ 1] = dist[x1][y1][op] + 1;
                    que.push({lx, ly, op ^ 1});
                }
            }
        }
    }

    cout << "-1\n";

}
signed main() {
    ios::sync_with_stdio(0);
    cin.tie(0);
    cout.tie(0);
    int _ = 1;
    //cin >> _;
    while(_--) {
        solve();
    }
}