using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace ConsoleApp133
{
    class Program
    {
        static void Main(string[] args)
        {
            string[] str = Console.ReadLine().Split();
            int a = int.Parse(str[0]);
            int b = int.Parse(str[1]);
            int c = int.Parse(str[2]);
            int d = int.Parse(str[3]);
            if (c / a >= d / b)
            {
                Console.WriteLine(d * (a + b / b));
            }
            else
            {
                Console.WriteLine(c * (a + b / a));
            }
        }
    }
}