#include <bits/stdc++.h>

#define rep(i,s,n) for(int i = int(s); i < int(n); i++)
#define rrep(i,s,n) for(int i = int(n) - 1; i >= int(s); i--)
#define all(v) (v).begin(), (v).end()

using ll = long long;
using ull = unsigned long long;
using ld = long double;

template<class T>
bool chmin(T &a, T b) {
    if(a <= b) return false;
    a = b;
    return true;
}

template<class T>
bool chmax(T &a, T b) {
    if(a >= b) return false;
    a = b;
    return true;
}

using namespace std;

int main() {
    std::cin.tie(nullptr);
    std::ios::sync_with_stdio(false);
    int x, y, p, q; cin >> x >> y >> p >> q;
    vector tansaku(2, vector(x+1, vector<bool>(y+1)));
    vector dep(2, vector(x+1, vector<int>(y+1, 1e9)));
    dep[0][0][0] = 0;
    tansaku[0][0][0] = 1;
    deque<tuple<int,int,int>> mada;
    mada.push_back(make_tuple(0, 0, 0));
    int ans = 1e9;
    while(!mada.empty()){
        // i ... sheep
        // j ... wolf
        auto [typ, i, j] = mada.front();
        if (i == x){
            ans = dep[typ][i][j];
            break;
        }
        //cout << typ << ' ' << i << ' ' << j << ' ' << dep[typ][i][j] << endl;
        mada.pop_front();
        if (typ == 0){
            int li = x-i;
            int lj = y-j;
            rep(s,0,p+1){
                if (s>li) break;
                rep(t,0,p+1){
                    if (t>lj) break;
                    if (s+t>p) break;

                    // joken
                    if (i>0 && j>i+q) continue;
                    if (li-s>0 && lj-t>li-s+q) continue;

                    if (tansaku[1][i+s][j+t]) continue;
                    tansaku[1][i+s][j+t] = 1;
                    dep[1][i+s][j+t] = dep[0][i][j] + 1;
                    mada.push_back(make_tuple(1, i+s, j+t));
                }
            }
        }else{
            
            int li = x-i;
            int lj = y-j;
            rep(s,0,p+1){
                if (s>i) break;
                rep(t,0,p+1){
                    if (t>j) break;
                    if (s+t>p) break;

                    if (li>0 && lj>li+q) continue;
                    if (i-s>0 && j-t>i-s+q) continue;
                    
                    if (tansaku[0][i-s][j-t]) continue;
                    tansaku[0][i-s][j-t] = 1;
                    dep[0][i-s][j-t] = dep[1][i][j] + 1;
                    mada.push_back(make_tuple(0, i-s, j-t));
                }
            }
        }
    }

    if (ans > 1e8){
        cout << -1 << endl;
    }else{
        cout << ans << endl;
    }
}