#include<bits/stdc++.h>
using namespace std;
signed main(){
	int x,y,q,p;
	cin >> x >> y >> p >> q;
	int flag=1;
	if(p>=x){
		cout << 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{
		if(x+q>=y){
			//一次运走
			if(p>=x){
				cout << 1;
				return 0;
			}
			//一次运不走
			else{
				int duo=x+q-y;
				int zs=(y-duo)*2+duo;
				cout << (zs+p-1)/p;
				return 0;
			} 
		}
		else flag=0;
	}
	if(!flag) cout << -1; 
	return 0;
}