21

using System;

namespace Unknown
{
    class Program
    {

        static void Main(string[] args)
        {
            int first = int.Parse(Console.ReadLine());
            int second = int.Parse(Console.ReadLine());
            int third = int.Parse(Console.ReadLine());
            int forth = int.Parse(Console.ReadLine());
            int sum = first + second;
            sum /= third;
            sum *= forth;
            Console.WriteLine(sum);

        }
    }
}