#include <bits/stdc++.h>
using namespace std;

#define fi first
#define se second
#define pb push_back
#define Mp make_pair
#define SZ(x) (int((x).size()))

typedef long long ll;
typedef double db;
typedef vector<int> vi;
typedef pair<int, int> pii;

void solve() {
    int n, m, p, q;
    cin >> n >> m >> p >> q;
    vector f(n + 1, vector(m + 1, vi(2, 1e9)));
    vector vis(n + 1, vector(m + 1, vi(2, 0)));
    f[0][0][0] = 0;
    queue<array<int, 3>> qe;
    qe.push({0, 0, 0});
    auto valid = [&](int a, int b) {
        return b <= a + q || a == 0;
    };
    while(SZ(qe)) {
        auto [x, y, ty] = qe.front(); qe.pop();
        if(vis[x][y][ty]) continue;
        // cerr << x << ' ' << y << ' ' << ty << '\n';
        vis[x][y][ty] = 1;
        if(ty) { // right
            int a = x, b = y;
            for(int i = 0; i <= a; i++)
                for(int j = 0; j <= b && i + j <= p; j++) {
                    if(!valid(a - i, b - j)) continue;
                    if(f[x - i][y - j][0] > f[x][y][1] + 1) {
                        f[x - i][y - j][0] = f[x][y][1] + 1;
                        qe.push({x - i, y - j, 0});
                    }
                }
        } else {
            int a = n - x, b = m - y;
            for(int i = 0; i <= a; i++)
                for(int j = 0; j <= b && i + j <= p; j++) {
                    if(!valid(a - i, b - j)) continue;
                    if(f[x + i][y + j][1] > f[x][y][0] + 1) {
                        f[x + i][y + j][1] = f[x][y][0] + 1;
                        qe.push({x + i, y + j, 1});
                    }
                }
        }
    }
    // cout << f[2][1][1] << ' ' << f[2][1][0] << ' ' << f[4][1][1] << '\n';
    int ans = 1e9;
    for(int i = 0; i <= m; i++)
        ans = min(ans, f[n][i][1]);
    if(ans == 1e9) ans = -1;
    cout << ans << '\n';
}
signed main() {
    ios::sync_with_stdio(false), cin.tie(0), cout.tie(0);
    int t = 1;
    // cin >> t;
    while(t--) solve();
    return 0;
}