using System;
using System.Collections.Generic;
using System.Linq;
using System.Numerics;
using static System.Math;

namespace SortItems
{
    class Program
    {
        static void Main(string[] args)
        {
            var n = Console.ReadLine().Split().Select(double.Parse).ToArray();
            double a = n[0];
            double b = n[1];
            double x = n[2];
            double y = n[3];

            if (x * b / a > y)
            {
                Console.WriteLine(y + (y * (a / b)));
            }
            else
            {
                Console.WriteLine(x + (x * (b / a)));
            }
            

        }
    }
}