#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
#define pii pair<int,int>
#define fs first
#define sc second
#define pb push_back
#define SZ(X) (int)((X).size())
#define lb(i) ((i)&-(i))
#define cl(i) memset(i,0,sizeof(i))
const int N=6.1e3;
int solve(int x,int y,int p,int q,int a,int b){// x y 对应左岸羊，狼；a b 对应右岸羊，狼
    int side=0,nw=0,cnt=0;
    for(;x>0;side^=1){
        cnt++;
        if(side==0){
            if(x<=p-nw)return cnt;
            while(nw<p){
                while(nw<p&&y-x<q)x--,a++,nw++;
                if(nw<p)y--,b++,nw++;
            }
            if(y-x>q)return 1e9;
            nw=0;
        }
        if(side==1){
            //x>0
            while(a>0&&nw<p&&b-a>q)b--,y++,nw++;
            //assert(a==0||b-a<=q);////
            if(!(a==0||b-a<=q))return 1e9;
            nw=0;
        }
    }
    return cnt;
}
int main(){
    int x,y,p,q;
    cin>>x>>y>>p>>q;
    int mn=solve(x,y,p,q,0,0);
    /*
    for(int i=1;i<=p*2&&y-i>=0;i++){
        if(y-min(i,p)-x<=q)mn=min(mn,solve(x,y-i,p,q,0,i)+2*((i-1)/p+1));
        //if(mn<1e9)cerr<<i<<" "<<mn<<"\n";
    }*/
    if(y-p-x<=q){
        mn=min(mn,solve(x,y-p,p,q,0,p)+2);
        mn=min(mn,solve(x,y-2*p,p,q,0,2*p)+4);
    }
    if(mn>=1e9)puts("-1");
    else printf("%d\n",mn);
    return 0;
}