#include <iostream>
#include <cstdio>
#include <cstdlib>
using namespace std;

int main(void)
{
	freopen("bs.in","r",stdin);
	freopen("bs.out","w",stdout);
	int c0,cx,m0,mx,t0,tx,temp;
	while (scanf("%d %d %d %d\n",&c0,&cx,&m0,&mx)==4)
	{
		if (m0==0&&mx==0)
		{
			printf("0\n");
		}
		else if (m0==0)
		{
			printf("%d\n",min(cx,mx));
		}
		else if (mx==0)
		{
			printf("%d\n",min(c0,m0));
		}
		else
		{
			
			if (c0%m0)
				t0=c0/m0+1;
			else
				t0=c0/m0;
			if (cx%mx)
				tx=cx/mx+1;
			else
				tx=cx/mx;
			
			temp=c0+cx;
			if (c0>=tx-1&&c0<=cx+1)
			{
				printf("%d\n",temp);
			}
			else if (t0>=tx-1&&t0<=cx+1)
			{
				printf("%d\n",temp);
			}
			else if (cx>=t0-1&&cx<=c0+1)
			{
				printf("%d\n",temp);
			}
			else if (tx>=t0-1&&tx<=c0+1)
			{
				printf("%d\n",temp);
			}
			else
			{
				if (tx>c0)
				{
					temp=c0+(c0+1)*mx;
					printf("%d\n",temp);
				}
				else if (t0>cx)
				{
					temp=cx+(cx+1)*m0;
					printf("%d\n",temp);
				}
			}
		}
	}
	return(0);
}