#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 main(){
    int x,y,p,q,cnt=0;
    cin>>x>>y>>p>>q;
    int a=0,b=0,side=0,nw=0;
    for(;x>0;side^=1){
        cnt++;
        if(side==0){
            if(x<=p-nw)return printf("%d\n",cnt),0;
            while(nw<p){
                while(nw<p&&y-x<=q)x--,a++,nw++;
                if(nw<p)y--,b++,nw++;
            }
            if(y-x>q)return puts("-1"),0;
            nw=0;
        }
        if(side==1){
            //x>0
            while(a>0&&nw<p&&b-a>q)b--,y++,nw++;
            assert(a==0||b-a<=q);////
            nw=0;
        }
    }
    printf("%d\n",cnt);
    cerr<<"#";
    return 0;
}