#include <bits/stdc++.h>
#define int long long
using namespace std;
using ll = long long;
using LD = long double;
using PII = pair<int, int>;
const int N = 1e3 + 10;
const int inf = 1e18;
const double eps = 1e-10;
const int mod = 1e9 + 7;
int n, _, m, k, z, p, qx, ans;
bool vis[N][N];
int dis[N][N][2];
bool check(int x, int y, int x2, int y2, int z)
{
    if (x < 0 || y < 0 || x2 < 0 || y2 < 0 || x > n || y > m || x2 > n || y2 > m)
        return false;
    if (!z)
        return (x == 0 || x + qx >= y);
    else
        return (x2 == 0 || x2 + qx >= y2);
}
void bfs()
{
    for (int i = 0; i <= n; i++)
    {
        for (int j = 0; j <= m; j++)
        {
            dis[i][j][0] = dis[i][j][1] = inf;
        }
    }
    queue<array<int, 6>> q;
    q.push({n, m, 0, 0, 0, 0});
    while (!q.empty())
    {
        auto [x, y, x2, y2, z, w] = q.front();
        // cout << x << " " << y << " " << x2 << " " << y2 << " " << z << " " << w << ": " << endl;
        q.pop();
        if (dis[x2][y2][z] != inf)
            continue;
        dis[x2][y2][z] = w;
        for (int i = 0; i <= p; i++)
        {
            for (int j = 0; j <= p - i; j++)
            {
                int nx, ny, nx2, ny2, nz = 1 - z, nw = w + 1;
                if (z)
                {
                    nx = x + i, ny = y + j;
                    nx2 = x2 - i, ny2 = y2 - j;
                }
                else
                {
                    nx = x - i, ny = y - j;
                    nx2 = x2 + i, ny2 = y2 + j;
                }
                // if (x == 3 && y == 3 && x2 == 0 && y2 == 2 && z == 0 && w == 2)
                //     cout << nx << " " << ny << " " << nx2 << " " << ny2 << " " << nz << " " << nw << endl;

                if (!check(nx, ny, nx2, ny2, z))
                    continue;

                q.push({nx, ny, nx2, ny2, nz, nw});
            }
        }
    }
    // if (x == 2 && y == 2 && x2 == 1 && y2 == 3 && z == 1 && w == 3)
    //     cout << nx << " " << ny << " " << nx2 << " " << ny2 << " " << nz << " " << nw << endl;

    int minn = inf;
    for (int i = 0; i <= m; i++)
    {
        minn = min(minn, dis[n][i][0]);
    }
    if (minn == inf)
        cout << -1 << endl;
    else
        cout << minn - 1 << endl;
}
void solve()
{
    cin >> n >> m >> p >> qx;
    bfs();
}
signed main()
{
    ios::sync_with_stdio(0);
    cin.tie(0);
    cout << fixed << setprecision(10);
    _ = 1;
    // cin >> _;
    while (_--)
    {
        solve();
    }
}