#include<bits/stdc++.h>
using namespace std;
#define x first
#define y second
typedef pair<int,int> PII;
typedef long long ll;
typedef unsigned long long ull;
typedef unsigned int uint;
typedef vector<string> VS;
typedef vector<int> VI;
typedef vector<vector<int>> VVI;
vector<int> vx;
inline void divide() {sort(vx.begin(),vx.end());vx.erase(unique(vx.begin(),vx.end()),vx.end());}
//inline int mp(int x) {return upper_bound(vx.begin(),vx.end(),x)-vx.begin();}
inline int log_2(int x) {return 31-__builtin_clz(x);}
inline int popcount(int x) {return __builtin_popcount(x);}
inline int lowbit(int x) {return x&-x;}
inline ll Lsqrt(ll x) { ll L = 1,R = 2e9;while(L + 1 < R){ll M = (L+R)/2;if(M*M <= x) L = M;else R = M;}return L;}
void solve()
{
   int x,y,p,q;
   cin>>x>>y>>p>>q;
   if(y<=x+q)
   {
        ll ans=0;
        if(x<=p) ans=1;
        else{
            int qu = (x-(y-q));
            ans+=qu/q*2;
            x-=qu/q*2;
            y-=qu/q*2;
            int left = qu%(qu/q*q),pp;
            x-=left;
            if(p>x) 
            {
                x=0;
                ans++;
            }else{
                pp-=left;
                x-=pp/2;
                y-=pp/2;
                ans+=2;
            }
            if(x>p)
            {   
                x-=p;
                ans+=(x+p-1)/p;
            }
            if(x>0)
            ans+=1;
        }
        cout<<ans<<endl;
   }else{
        long long ans=0;
        if(y-p>x+q)
        {
            cout<<-1<<endl;
            return ;
        }
        ll pp =p;
        ll qu = max(0,(y-(x+q))),rightw=0,righty=0;
        pp -= qu;
        rightw+=qu;
        int cnt=0;
        while(x>0&&pp>=2)
        {
            if(++cnt>=101) break;
        x-=1;
        y-=1;
        rightw+=1;
        righty+=1;
        pp-=2;
        }
        ans+=2;
        if(righty)
        {
            pp=q-qu;
        }else{
            x-=p/2;
            y-=p/2;
            rightw+=p/2;
            righty+=p/2;
            pp =p-max(rightw-righty+q,0ll);
            ans+=2;
        }
        if(x>p)
        { x-=p;
        ans+=(x+pp-1)/pp;
        }
        ans+=1;
        cout<<ans<<endl;
   }
}
int main()
{
	//ios::sync_with_stdio(false);
	//cin.tie(0);
	int T=1;
	//cin>>T;
	while(T--)
	{
		solve();
	}
}