#include<iostream>
#include<cstdio>
#include<cstdlib>
#include<cmath>
#include<cstring>
#include<algorithm>

using namespace std;

int Coo,Cox,Mao,Max;

int main()
{
	freopen("bs.in","r",stdin);
	freopen("bs.out","w",stdout);
	while(scanf("%d%d%d%d",&Coo,&Cox,&Mao,&Max)!=EOF)
	{
		if(!Coo||!Mao)
		{
			printf("%d\n",min(Cox,Max));
			continue;
		}
		if(!Cox||!Max)
		{
			printf("%d\n",min(Coo,Mao));
			continue;
		}
		int l1,l2;
		if(!(Coo%Mao))
			l1=Coo/Mao;
		else
			l1=Coo/Mao+1;
		if(!(Cox%Max))
			l2=Cox/Max;
		else
			l2=Cox/Max+1;
		if(Coo<l2-1||Cox<l1-1)
		{
			if(Coo<l2-1)
				printf("%d\n",(Coo+1)*Max+Coo);
			if(Cox<l1-1)
				printf("%d\n",(Cox+1)*Mao+Cox);
		}
		else
			printf("%d\n",Coo+Cox);
	}
	return 0;
}