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

using ll = long long;
#define rep(i, n) for (ll i = 0; i < n; i++)
#define rep2(i, l, r) for (ll i = l; i < r; i++)

using vi = vector<int>;
using vvi = vector<vi>;
using vll = vector<ll>;

#define all(A) A.begin(), A.end()
#define elif else if
using pii = pair<ll, ll>;

bool chmin(auto &a, const auto &b) { return a > b ? a = b, 1 : 0; }
bool chmax(auto &a, const auto &b) { return a < b ? a = b, 1 : 0; }

struct IOSetup {
  IOSetup() {
    cin.tie(0);
    ios::sync_with_stdio(0);
  }
} iosetup;

template <class T>
void print(vector<T> a) {
  for (auto x : a) cerr << x << ' ';
  cout << endl;
}

void print(auto x) { cout << x << endl; }

template <class Head, class... Tail>
void print(Head &&head, Tail &&...tail) {
  cout << head << ' ';
  print(forward<Tail>(tail)...);
}

int main() {
  int inf = 1 << 30;
  int x, y, p, q;
  cin >> x >> y >> p >> q;
  vector<vvi> dp(2, vvi(x + 1, vi(y + 1, inf)));
  dp[0][x][y] = 0;
  deque<array<int, 3>> todo;
  todo.push_back({0, x, y});
  while (!todo.empty()) {
    auto [t, a, b] = todo.front();
    todo.pop_front();
    if (t == 1 && a == 0) {
      cout << dp[t][a][b] << endl;
      exit(0);
    }
    int sgn = -1;
    if (t == 1) sgn = 1;
    rep(da, p + 1) {
      rep(db, p - da + 1) {
        int la = a + sgn * da, lb = b + sgn * db;
        if (0 <= la && la <= x && 0 <= lb && lb <= y) {
          int ra = x - la, rb = y - lb;
          bool flag = true;
          if (t == 0 && la > 0 && la + q < lb) flag = false;
          if (t == 1 && ra > 0 && ra + q < rb) flag = false;
          if (flag) {
            if (dp[t ^ 1][la][lb] == inf) {
              dp[t ^ 1][la][lb] = dp[t][a][b] + 1;
              todo.push_back({t ^ 1, la, lb});
            }
          }
        }
      }
    }
  }
  cout << -1 << endl;
}