#include<iostream>
#include<cstdio>
#include<cstdlib>
using namespace std;
int m,n,q,w;
int main()
{
	freopen ("bs.in","r",stdin);
	freopen ("bs.out","w",stdout);
	while ((cin>>m>>n>>q>>w))
	{
		if (q==0)
		{
			cout<<w<<endl;
			continue;
		}
		if (w==0)
		{
			cout<<q<<endl;
			continue;
		}
		q=min(q,n);
		w=min(w,m);
		if ((n/q)>(m/w))
		{
			cout<<(m+q*((m/w)+1))<<endl;
			continue;
		}
		if ((m/w)>(n/q))
		{
			cout<<(n+w*((n/q)+1))<<endl;
			continue;
		}
		if ((n/q)==(m/w))
		{
			cout<<((n/q)*q+(m/w)*w+max(n%q,m%w))<<endl;
			continue;
		}
	}
	return 0;
}