#include <map>
#include <set>
#include <array>
#include <cmath>
#include <deque>
#include <bitset>
#include <random>
#include <string>
#include <vector>
#include <cassert>
#include <complex>
#include <iomanip>
#include <iostream>
#include <algorithm>
#include <unordered_map>
#include <unordered_set>

using namespace std;

const int MAXN = 110;
const int INF = 1e9;
int dist[MAXN][MAXN][2];


int main() {
#ifdef DEBUG
  freopen("input.txt", "r", stdin);
  freopen("output.txt", "w", stdout);
#else
  ios::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
#endif

  int x, y, p, q;
  cin >> x >> y >> p >> q;
  for (int a = 0; a <= x; ++a) {
    for (int b = 0; b <= y; ++b)
      dist[a][b][0] = dist[a][b][1] = INF;
  }
  deque<pair<pair<int, int>, int>> Q;
  Q.emplace_back(make_pair(x, y), 0);
  dist[x][y][0] = 0;
  while (!Q.empty()) {
    auto [ab, t] = Q.front();
    auto [a, b] = ab;
    if (a == 0 && t == 1) {
      cout << dist[a][b][t] << '\n';
      return 0;
    }
    Q.pop_front();
    int c = x - a, d = y - b;
    if (t == 0) {
      for (int take1 = 0; take1 <= a; ++take1) {
        for (int take2 = 0; take2 <= b && take1 + take2 <= p; ++take2) {
          if (a - take1 && b - take2 > a - take1 + q)
            continue;
          int nxt_a = a - take1, nxt_b = b - take2;
          if (dist[nxt_a][nxt_b][1] == INF) {
            dist[nxt_a][nxt_b][1] = dist[a][b][t] + 1;
            Q.emplace_back(make_pair(nxt_a, nxt_b), 1);
          }
        }
      }
    } else {
      for (int take1 = 0; take1 <= c; ++take1) {
        for (int take2 = 0; take2 <= d && take1 + take2 <= p; ++take2) {
          if (c - take1 && d - take2 > c - take1 + q)
            continue;
          int nxt_a = a + take1, nxt_b = b + take2;
          if (dist[nxt_a][nxt_b][0] == INF) {
            dist[nxt_a][nxt_b][0] = dist[a][b][t] + 1;
            Q.emplace_back(make_pair(nxt_a, nxt_b), 0);
          }
        }
      }
    }
  }
  cout << -1 << '\n';
  return 0;
}