#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
#include <ext/rope>

#define ll long long
#define pb push_back
#define f first
#define s second
#define sz(x) (int)(x).size()
#define all(x) x.begin(), x.end()
#define rall(x) x.rbegin(), x.rend()
#define ios ios_base::sync_with_stdio(false);cin.tie(NULL)
#define ld long double
#define li __int128

using namespace std;
using namespace __gnu_pbds;
using namespace __gnu_cxx;

mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());
template<class T> using ordered_set = tree<T, null_type, less<T>, rb_tree_tag,tree_order_statistics_node_update>; ///find_by_order(),order_of_key()
template<int D, typename T>struct vec : public vector<vec<D - 1, T>> {template<typename... Args>vec(int n = 0, Args... args) : vector<vec<D - 1, T>>(n, vec<D - 1, T>(args...)) {}};
template<typename T>struct vec<1, T> : public vector<T> {vec(int n = 0, T val = T()) : vector<T>(n, val) {}};
template<class T1,class T2> ostream& operator<<(ostream& os, const pair<T1,T2>& a) { os << '{' << a.f << ", " << a.s << '}'; return os; }
template<class T> ostream& operator<<(ostream& os, const vector<T>& a){os << '{';for(int i=0;i<sz(a);i++){if(i>0&&i<sz(a))os << ", ";os << a[i];}os<<'}';return os;}
template<class T> ostream& operator<<(ostream& os, const deque<T>& a){os << '{';for(int i=0;i<sz(a);i++){if(i>0&&i<sz(a))os << ", ";os << a[i];}os<<'}';return os;}
template<class T> ostream& operator<<(ostream& os, const set<T>& a) {os << '{';int i=0;for(auto p:a){if(i>0&&i<sz(a))os << ", ";os << p;i++;}os << '}';return os;}
template<class T> ostream& operator<<(ostream& os, const set<T,greater<T> >& a) {os << '{';int i=0;for(auto p:a){if(i>0&&i<sz(a))os << ", ";os << p;i++;}os << '}';return os;}
template<class T> ostream& operator<<(ostream& os, const multiset<T>& a) {os << '{';int i=0;for(auto p:a){if(i>0&&i<sz(a))os << ", ";os << p;i++;}os << '}';return os;}
template<class T> ostream& operator<<(ostream& os, const multiset<T,greater<T> >& a) {os << '{';int i=0;for(auto p:a){if(i>0&&i<sz(a))os << ", ";os << p;i++;}os << '}';return os;}
template<class T1,class T2> ostream& operator<<(ostream& os, const map<T1,T2>& a) {os << '{';int i=0;for(auto p:a){if(i>0&&i<sz(a))os << ", ";os << p;i++;}os << '}';return os;}
int ri(){int x;scanf("%i",&x);return x;}
void rd(int&x){scanf("%i",&x);}
void rd(long long&x){scanf("%lld",&x);}
void rd(double&x){scanf("%lf",&x);}
void rd(long double&x){scanf("%Lf",&x);}
void rd(string&x){cin>>x;}
void rd(char*x){scanf("%s",x);}
template<typename T1,typename T2>void rd(pair<T1,T2>&x){rd(x.first);rd(x.second);}
template<typename T>void rd(vector<T>&x){for(T&p:x)rd(p);}
template<typename C,typename...T>void rd(C&a,T&...args){rd(a);rd(args...);}
//istream& operator>>(istream& is,__int128& a){string s;is>>s;a=0;int i=0;bool neg=false;if(s[0]=='-')neg=true,i++;for(;i<s.size();i++)a=a*10+s[i]-'0';if(neg)a*=-1;return is;}
//ostream& operator<<(ostream& os,__int128 a){bool neg=false;if(a<0)neg=true,a*=-1;ll high=(a/(__int128)1e18);ll low=(a-(__int128)1e18*high);string res;if(neg)res+='-';if(high>0){res+=to_string(high);string temp=to_string(low);res+=string(18-temp.size(),'0');res+=temp;}else res+=to_string(low);os<<res;return os;}

const int N=101;
int dp[N][N][2],X,Y,P,Q,oo=INT_MAX/3;
bool moze(int x,int y,bool side){
	x=X-x;
	y=Y-y;
	if(x==0)return 1;
	if(x+Q<y)return 0;
	return 1;
}
int calc(int x,int y,int side){
	if(!moze(x,y,side)){
		return oo;
	}
	printf("Moze %i %i %i\n",x,y,side);
	if(x==X&&side==1)return 0;
	if(dp[x][y][side]!=-1)return dp[x][y][side];
	dp[x][y][side]=oo;
	for(int a=0;a<=min(x,P);a++){
		for(int b=0;b<=min(y,P-a);b++){
			int x2=X-x;
			int y2=Y-y;
			x2+=a;
			y2+=b;
			int o=calc(x2,y2,side^1);
			dp[x][y][side]=min(dp[x][y][side],o+1);
		}
	}
	printf("%i %i %i: %i\n",x,y,side,dp[x][y][side]);
	return dp[x][y][side];
}
int main()
{
	memset(dp,-1,sizeof dp);
	rd(X,Y,P,Q);
	queue<pair<pair<int,int>,int>> q;
	dp[X][Y][0]=0;
	q.push({{X,Y},0});
	while(q.size()){
		auto tr=q.front();
		q.pop();
		int x=tr.f.f,y=tr.f.s,side=tr.s;
		for(int a=0;a<=min(x,P);a++){
			for(int b=0;b<=min(y,P-a);b++){
				int x2=X-x;
				int y2=Y-y;
				x2+=a;
				y2+=b;
				if(moze(x2,y2,side^1)){
					if(dp[x2][y2][side^1]==-1){
						dp[x2][y2][side^1]=dp[x][y][side]+1;
						q.push({{x2,y2},side^1});
					}
				}
			}
		}
	}
	int ans=INT_MAX;
	for(int o=0;o<=Y;o++){
		if(dp[X][o][1]!=-1){
			ans=min(ans,dp[X][o][1]);
		}
	}
	if(ans==INT_MAX)ans=-1;
	printf("%i\n",ans);
	return 0;
}