#include<bits/stdc++.h>
#define Gc() getchar()
#define Me(x,y) memset(x,y,sizeof(x))
#define Mc(x,y) memcpy(x,y,sizeof(x))
#define d(x,y) ((m)*(x-1)+(y))
#define R(n) (rnd()%(n)+1)
#define Pc(x) putchar(x)
#define LB lower_bound
#define UB upper_bound
#define fi first
#define se second
#define eb emplace_back
#define all(x) x.begin(),x.end()
using namespace std;using ll=long long;using db=double;using lb=long db;using ui=unsigned;using ull=unsigned long long;using pii=pair<int,int>;
const int N=100+5,M=4e3+5,K=1000+5,mod=1e9+7,Mod=mod-1;const db eps=1e-9;const int INF=1e9+7;mt19937 rnd(263082);
#define Tp template<typename T>
#define Ts template<typename T,typename... Ar>
namespace Debug{
	Tp void _debug(char* f,T t){cerr<<f<<'='<<t<<endl;}
	Ts void _debug(char* f,T x,Ar... y){while(*f!=',') cerr<<*f++;cerr<<'='<<x<<",";_debug(f+1,y...);}
	#ifdef LOCAL
	#define gdb(...) _debug((char*)#__VA_ARGS__,__VA_ARGS__)
	#else 
	#define gdb(...) void()
	#endif
}using namespace Debug;
int X,Y,P,Q;
int d[N][N][2];
void Solve(){
	int i,j;scanf("%d%d%d%d",&X,&Y,&P,&Q);
	Me(d,0x3f);d[X][Y][0]=0;
	queue<tuple<int,int,int> > q;
	q.emplace(X,Y,0);
	while(!q.empty()){
		int s,t,op;tie(s,t,op)=q.front();q.pop();
		if(op){
			for(int x=0;x<=X-s;x++) for(int y=0;y<=Y-t&&x+y<=P;y++) if((Y-t)-y<=(X-s)-x+Q||x==X-s){
				if(d[s+x][t+y][0]>d[s][t][1]+1) d[s+x][t+y][0]=d[s][t][1]+1,q.emplace(s+x,t+y,0);
			}
		}else{
			for(int x=0;x<=s;x++) for(int y=0;y<=t&&x+y<=P;y++) if(t-y<=s-x+Q||x==s){
				if(d[s-x][t-y][1]>d[s][t][0]+1) d[s-x][t-y][1]=d[s][t][0]+1,q.emplace(s-x,t-y,1);
			}
		}
	}
	// gdb(d[2][3][0]);
	int ans=INF;
	for(i=0;i<=100;i++) ans=min(ans,d[0][i][1]);
	if(ans>=INF) puts("-1");
	else printf("%d\n",ans);
}
int main(){
	int t=1;
	// scanf("%d",&t);
	while(t--) Solve();
	cerr<<clock()*1.0/CLOCKS_PER_SEC<<'\n';
}