//by 72
#include<bits/stdc++.h>
#define F(i, a, b) for(int i = a; i <= b; i ++)
#define Fd(i, a, b) for(int i = a; i >= b; i --)
#define pb push_back
#define pii pair<int, int>
#define fi first
#define se second
#define int long long
using namespace std;

const int mod = 998244353;
const int N = 105;
const int inf = 1e18;
typedef array<int, 3> a3; 
typedef long long ll;

int x, y, p, q, f[N][N][2], vst[N][N][2];

bool judge(int a, int b) {
    if(a < 0 || b < 0) return false;
    if(! a || ! b) return true;
    return a + q >= b;
}

void sol() {
    cin >> x >> y >> p >> q;
    F(i, 0, x) F(j, 0, y) F(k, 0, 1) f[i][j][k] = inf;
    f[0][0][0] = 0;
    queue<a3> q;
    q.push({0, 0, 0});
    while(q.size()) {
        auto [u, v, w] = q.front();
        if(vst[u][v][w]) continue;
        vst[u][v][w] = 1;
        int uu = x - u, vv = y - v;
        q.pop();
        F(a, 0, x) F(b, 0, y) {
            if(a + b > p) continue;
            // 这次船上载多少
            if(w == 0) {
                if(! judge(uu - a, vv - b)) continue;
                if(f[u + a][v + b][1] > f[u][v][w] + 1) {
                    q.push({u + a, v + b, 1});
                    f[u + a][v + b][1] = f[u][v][w] + 1;
                }
            } else {
                if(! judge(u - a, v - b)) continue;
                if(f[u - a][v - b][0] > f[u][v][w] + 1) {
                    q.push({u - a, v - b, 0});
                    f[u - a][v - b][0] = f[u][v][w] + 1;
                }
            }
        }
    }
    int res = inf;
    F(i, 0, y) res = min(res, f[x][i][1]);
    // cout << f[x][y][1] << "??\n";
    if(res == inf) cout << "-1\n";
    else cout << (res + 1) / 2 << "\n";
}

signed main() {
    ios::sync_with_stdio(false);
    cin.tie(0);
    int t = 1;
    // cin >> t;
    F(i, 1, t) sol();
    return 0;
}
//sldl