#pragma GCC optimize("O2")
#include <algorithm>
#include <array>
#include <bit>
#include <bitset>
#include <cassert>
#include <cctype>
#include <cfenv>
#include <cfloat>
#include <chrono>
#include <cinttypes>
#include <climits>
#include <cmath>
#include <compare>
#include <complex>
#include <concepts>
#include <cstdarg>
#include <cstddef>
#include <cstdint>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <deque>
#include <fstream>
#include <functional>
#include <initializer_list>
#include <iomanip>
#include <ios>
#include <iostream>
#include <istream>
#include <iterator>
#include <limits>
#include <list>
#include <map>
#include <memory>
#include <new>
#include <numbers>
#include <numeric>
#include <ostream>
#include <queue>
#include <random>
#include <ranges>
#include <set>
#include <span>
#include <sstream>
#include <stack>
#include <streambuf>
#include <string>
#include <tuple>
#include <type_traits>
#include <variant>

//#define int ll
#define INT128_MAX (__int128)(((unsigned __int128) 1 << ((sizeof(__int128) * __CHAR_BIT__) - 1)) - 1)
#define INT128_MIN (-INT128_MAX - 1)

namespace R = std::ranges;
namespace V = std::views;

using namespace std;

using ll = long long;
using ull = unsigned long long;
using pii = pair<int, int>;
using pll = pair<long long, long long>;
using tiii = tuple<int, int, int>;
using ldb = long double;
//#define double ldb

template<class T>
ostream& operator<<(ostream& os, const pair<T, T> pr) {
  return os << pr.first << ' ' << pr.second;
}
template<class T, size_t N>
ostream& operator<<(ostream& os, const array<T, N> &arr) {
  for(const T &X : arr)
    os << X << ' ';
  return os;
}
template<class T>
ostream& operator<<(ostream& os, const vector<T> &vec) {
  for(const T &X : vec)
    os << X << ' ';
  return os;
}

int d[101][101][2];

signed main() {
  ios::sync_with_stdio(false), cin.tie(NULL);

  int x0, y0, p, q; cin >> x0 >> y0 >> p >> q;

  for(int x = 0; x <= x0; x++)
    for(int y = 0; y <= y0; y++)
      d[x][y][0] = d[x][y][1] = INT_MAX;

  d[x0][y0][0] = 0;
  queue<array<int, 3>> qq;
  qq.push({x0, y0, 0});
  while(!qq.empty()) {
    auto [x, y, z] = qq.front(); qq.pop();
    if (z == 0) {
      for(int a = 0; a <= x and a <= p; a++)
        for(int b = 0; b <= y and a + b <= p; b++)
          if (d[x - a][y - b][1] == INT_MAX and (x - a == 0 or y - b <= x - a + q)) {
            d[x - a][y - b][1] = d[x][y][z] + 1;
            qq.push({x - a, y - b, 1});
          }
    } else {
      for(int a = 0; x + a <= x0 and a <= p; a++)
        for(int b = 0; y + b <= y0 and a + b <= p; b++)
          if (d[x + a][y + b][0] == INT_MAX and (x0 - (x + a) == 0 or y0 - (y + b) <= x0 - (x + a) + q)) {
            d[x + a][y + b][0] = d[x][y][z] + 1;
            qq.push({x + a, y + b, 0});
          }
    }
  }

  int ans = INT_MAX;
  for(int j = 0; j <= y0; j++)
    ans = min(ans, d[0][j][1]);

  if (ans == INT_MAX)
    cout << -1 << '\n';
  else
    cout << ans << '\n';

  return 0;
}