#include<bits/stdc++.h>
using namespace std;
const int SIZE = 2e5+10;
int x,y,p,q,xl,xr,yl,yr;

int main(){
    scanf("%lld%lld%lld%lld",&x,&y,&p,&q);
    int ans=0;
    xl=x,xr=0,yl=y,yr=0;
    while(xl)
    {
        if(xl<=p){ans++;break;}
        else if(yl-q>xl-p)
        {
            ans++;
            int k=yl-xl-q;
            if(k>p){printf("-1");return 0;}
            int wolf =(k+p+1)/2;
            yl-=wolf;
            yr=wolf;
            xl-=p-wolf;
            xr+=p-wolf;
        }
        else
        {
            ans++;
            xl-=p;
            xr+=p;
        }
        ans++;
        if(yr-xr-q>0&&xr>0)
        {
            yl+=yr-xr-q;
            yr-=yr-xr-q;
        }
    }
    printf("%lld",ans);
    return 0;
}