#include<bits/stdc++.h>
using namespace std;
#define int long long
#define pb push_back
#define F first
#define S second
int x, y, p, q;
bool check(int i, int j, int k){
	// if(i == 3 && j == 3 && k == 1) cout << "entered\n";
	 if(i < 0 || i > x || j < 0 || j > y) return 0;
	 if(j > i + q && k == 1 && i > 0) return 0;
	 if(y - j > x - i + q && k == 0 && i < x) return 0;
//	 if(i == 3 && j == 3 && k == 1) cout << "correct\n";
	 return 1;
}
struct node{
	int s, w, side;
};

vector<node> adj[105][105][2];
int d[105][105][2];
node par[105][105][2];
signed main(){
//	x = 3, y = 5, p = 2, q = 0;
	cin >> x >> y >> p >> q;
	for(int i = 0; i <=x; i++){
		for(int j = 0; j <= y; j++){
			  d[i][j][0] = d[i][j][1] = 1e9;
		      for(int dif = 100; dif >= -100; dif--){
		      	  int a = (p - dif)/2;
                  int b = a + dif;
                 // if(dif == 2) cout << a << " " << b << '\n';
		      	  if(a < 0 || a > i || b < 0 || b > j || a + b > p) continue;
		      	 // cout << "i , j:" << i << " " << j << '\n';
		      	  if(check(i - a, j - b, 1)) adj[i][j][0].push_back({i - a, j - b, 1});
			  }
			  for(int k = 0; k <=x - i; k++){
			  	  if(k <= p && check(i + k, j, 0)) adj[i][j][1].pb({i + k, j, 0});
			  }
			  for(int k = 0; k <=y - j; k++){
			  	  if(k <= p && check(i, j + k, 0)) adj[i][j][1].pb({i, j + k, 0});
			  }
		}
	}
	
	queue<node> q;
	d[x][y][0] = 0;
	q.push({x, y, 0});
	while(!q.empty()){
		auto it = q.front(); q.pop();
		for(auto next : adj[it.s][it.w][it.side]){
			if(d[it.s][it.w][it.side] + 1 < d[next.s][next.w][next.side]){
				 d[next.s][next.w][next.side] = d[it.s][it.w][it.side] + 1;
				 par[next.s][next.w][next.side] = it;
				 q.push(next);
			}
		}
	}


    int ans = 1e9;
    for(int j = 0; j <= y; j++){
    	ans = min(ans, d[0][j][1]);
	}
	if(ans == 1e9) cout << "-1\n";
	else cout << ans << '\n';
	
	
	
	
	   
}