#ifndef LOCAL
#define FAST_IO
#endif

// ============
#include <algorithm>
#include <array>
#include <bitset>
#include <cassert>
#include <cmath>
#include <cstring>
#include <iomanip>
#include <iostream>
#include <list>
#include <map>
#include <numeric>
#include <queue>
#include <random>
#include <set>
#include <stack>
#include <string>
#include <tuple>
#include <unordered_map>
#include <unordered_set>
#include <utility>
#include <vector>

#define OVERRIDE(a, b, c, d, ...) d
#define REP2(i, n) for (i32 i = 0; i < (i32)(n); ++i)
#define REP3(i, m, n) for (i32 i = (i32)(m); i < (i32)(n); ++i)
#define REP(...) OVERRIDE(__VA_ARGS__, REP3, REP2)(__VA_ARGS__)
#define PER(i, n) for (i32 i = (i32)(n) - 1; i >= 0; --i)
#define ALL(x) begin(x), end(x)

using namespace std;

using u32 = unsigned int;
using u64 = unsigned long long;
using i32 = signed int;
using i64 = signed long long;
using f64 = double;
using f80 = long double;

template <typename T>
using Vec = vector<T>;

template <typename T>
bool chmin(T &x, const T &y) {
    if (x > y) {
        x = y;
        return true;
    }
    return false;
}
template <typename T>
bool chmax(T &x, const T &y) {
    if (x < y) {
        x = y;
        return true;
    }
    return false;
}

#ifdef INT128

using u128 = __uint128_t;
using i128 = __int128_t;

istream &operator>>(istream &is, i128 &x) {
    i64 v;
    is >> v;
    x = v;
    return is;
}
ostream &operator<<(ostream &os, i128 x) {
    os << (i64)x;
    return os;
}
istream &operator>>(istream &is, u128 &x) {
    u64 v;
    is >> v;
    x = v;
    return is;
}
ostream &operator<<(ostream &os, u128 x) {
    os << (u64)x;
    return os;
}

#endif

[[maybe_unused]] constexpr i32 INF = 1000000100;
[[maybe_unused]] constexpr i64 INF64 = 3000000000000000100;
struct SetUpIO {
    SetUpIO() {
#ifdef FAST_IO
        ios::sync_with_stdio(false);
        cin.tie(nullptr);
#endif
        cout << fixed << setprecision(15);
    }
} set_up_io;
// ============

#ifdef DEBUGF
#else
#define DBG(x) (void) 0
#endif

int main() {
    i32 x, y, p, q;
    cin >> x >> y >> p >> q;
    
    Vec<Vec<Vec<i32>>> dist(2, Vec<Vec<i32>>(x + 1, Vec<i32>(y + 1, -1)));
    queue<tuple<i32, i32, i32>> que;
    dist[0][x][y] = 0;
    que.emplace(0, x, y);
    while (!que.empty()) {
        auto [side, a, b] = que.front();
        que.pop();
        i32 sheep[2] = {a, x - a};
        i32 wolf[2] = {b, y - b};
        REP(i, min(p, sheep[side]) + 1) {
            REP(j, min(p - i, wolf[side]) + 1) {
                sheep[side] -= i;
                sheep[side ^ 1] += i;
                wolf[side] -= j;
                wolf[side ^ 1] += j;
                if (sheep[side] + q >= wolf[side] || sheep[side] == 0) {
                    if (dist[side ^ 1][sheep[0]][wolf[0]] == -1) {
                        dist[side ^ 1][sheep[0]][wolf[0]] = dist[side][a][b] + 1;
                        que.emplace(side ^ 1, sheep[0], wolf[0]);
                    }
                }
                sheep[side] += i;
                sheep[side ^ 1] -= i;
                wolf[side] += j;
                wolf[side ^ 1] -= j;
            }
        }
    }
    DBG(dist);
    i32 ans = INF;
    REP(i, y + 1) {
        if (dist[1][0][i] != -1) chmin(ans, dist[1][0][i]);
    }
    if (ans == INF) cout << -1 << '\n';
    else cout << ans << '\n';
}