#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){
			flag=0;
		}
		else{
			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);
			if(yang==0 && q==0){
				if(x-1<=p){
					cout << 5; 
				}
				else cout << -1;
				return 0;
			}
			yang=0;
			cs+=2;
			int cw=p-lang;
			if(zuolang==zuoyang+q){
				//先狼
				if(zuoyang>p){
					cs+=((zuoyang-p)+cw-1)/cw*2+1;
				}
				else cs++;
			}
			else{
				//先羊
				if(zuoyang>p){
					cs+=((zuoyang-p)-1+cw-1)/cw*2+1;
				}
				else cs++;
			}
//			cout << zuolang << " " << zuoyang << " " << p <<   " " << cw << "\n";
		}
		if(flag) cout << cs;
	}
	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;
}