#include <bits/stdc++.h>
using namespace std;
#define ll long long
#define pii pair<int, int>
#define inf 0x3f3f3f3f
#define db double
#define il inline
#define endl '\n'
const int N = 2e5 + 5;
const int mod = 998244353;
int v[200][200][2];
struct node {
    int x, y, f, w;
};
void solve()
{
    int n, m, p, q;
    cin >> n >> m >> p >> q;
    queue<node> qq;
    qq.push({n, m, 0, 0});
    v[n][m][0] = 1;
    while (qq.size())
    {
        auto [x, y, f, w] = qq.front();
        qq.pop();
        if (f == 1)
        {
            if (x == n)
            {
                cout << w << '\n';
                return;
            }
        }
        if (f == 0)
        {
            if (x == 0)
            {
                cout << w << '\n';
                return;
            }
        }
        for (int i = 0; i <= p; i++) // 枚举运送的总数量
        {
            for (int j = 0; j <= i; j++) // 枚举运送羊的数量
            {
                int nx = x - j, ny = y - (i - j);
                if (nx < 0 || ny < 0) continue;
                if (nx + q < ny && nx != 0) continue;
                if (v[n - nx][m - ny][f ^ 1]) continue;
                v[n - nx][m - ny][f ^ 1] = 1;
                qq.push({n - nx, m - ny, f ^ 1, w + 1});
            }
        }
    } 
    cout << -1 << '\n';
}
signed main()
{
    // freopen("D:\\3022244240\\vscode\\txt\\in.txt", "r", stdin);
    // freopen("D:\\3022244240\\vscode\\txt\\out.txt", "w", stdout);
    // ios::sync_with_stdio(false);
    // cin.tie(0); cout.tie(0);
    int T = 1;
    // cin >> T;
    while (T--) solve();
    return 0;
}