#include<bits/stdc++.h>
using namespace std;
signed main(){
	int x,y,q,p;
	cin >> x >> y >> p >> q;
	if(p>=x){
		cout << 1;
		return 0;
	}
	if(q>=y){
		cout << (x+p-1)/p*2-1;
		return 0;
	}
	if(y>x+q){
		int cs=0;
		if(y>x+q+p){
			cout << -1;
			return 0;
		}
		else{
			int qf=-1;
			if(y!=x+q+p){
				int duo=y-x-q;
				int kong=p-duo;
				int lang=(kong+1)/2+duo;
				int yang=kong/2;
				int zuolang=y-lang,zuoyang=x-yang;
				lang=lang-min(lang,yang+q);
				yang=0;
				cs+=2;
				int cw=p-lang;
				if(zuolang==zuoyang+q){
					//先狼
					if(zuoyang>p){
						cs+=((zuoyang-p)*2+cw-1)/cw*2+1;
					}
					else cs++;
				}
				else{
					//先羊
					if(zuoyang>p){
						cs+=((zuoyang-p)*2-1+cw-1)/cw*2+1;
					}
					else cs++;
				}
//				cout << zuolang << " " << zuoyang << " " << p <<   " " << cw << "\n";
			}
			if(2*(x-p)>=p-q) qf=5;
			if(qf!=-1 && cs!=0){
				cout << min(qf,cs);
			}
			else if(qf!=-1){
				cout << qf;
			}
			else if(cs!=0){
				cout << cs;
			}
			else cout << -1;
		}
	}
	else{
		//一次运走
		if(p>=x){
			cout << 1;
			return 0;
		}
		//一次运不走
		else{
//			if(y>=q){
//				cout << (x+p-1)/p*2-1;
//				return 0;
//			}
			int duo=x-max(0,y-q);
			int yang=max(0,y-q);
			int cs=duo/p*2;
			int yu=duo%p;
			int flag1=1;
//			cout << duo << " " << yang << "\n";
			while(yang+yu>p){
				int dang=p-yu;
				yu=0;
				if(flag1){
					int yang1=dang/2;
					int lang1=(dang+1)/2;
					if(yang1!=lang1) flag1=0;
					yang-=yang1;
	//				cout << yang1 << "\n";
				}
				else{
					int lang1=dang/2;
					int yang1=(dang+1)/2;
					if(yang1!=lang1) flag1=1;
					yang-=yang1;
	//				cout << yang1 << "\n";
				}
				cs+=2;
			}
			if(yang+yu>0){
				cs++;
			}
			else cs--;
			cout << cs;
			return 0;
		}
	}
	return 0;
}