using System;
using System.Collections.Generic;
using System.Linq;

namespace YukiCoderNo236
{
    class Program
    {
        static void Main()
        {
            double[] buffer = LIB.IO.R<double>(' ');
            double per1 = buffer[0] * buffer[3];
            double per2 = buffer[1] * buffer[2];
            double ans = 0;
            if (per1 >= per2)
            {
                ans = buffer[2] + buffer[2] * buffer[1] / buffer[0];
            }
            else
            {
                ans = buffer[3] + buffer[3] * buffer[0] / buffer[1];
            }
            LIB.IO.W(ans);
            LIB.IO.WFLUSH();
        }
    }
}
namespace LIB
{
    public class IO
    {
        private const int WMAX = 1000;
        private static string WSTRING = "";
        public static T R<T>()
        {
            return (T)(Convert.ChangeType(R(), typeof(T)));
        }
        public static T[] R<T>(char splitter = ' ')
        {
            return R().Split(splitter).Select(v => UTILITY.PARSE<T>(v)).ToArray();
        }
        public static T[] R<T>(int length)
        {

            T[] ret = new T[length];
            for (int i = 0; i < length; i++)
            {
                ret[i] = R<T>();
            }
            return ret;
        }
        public static T[][] R<T>(int length, char splitter = ' ')
        {
            T[][] ret = new T[length][];
            for (int i = 0; i < length; i++)
            {
                ret[i] = R<T>(splitter);
            }
            return ret;
        }
        private static string R()
        {
            return Console.ReadLine();
        }
        public static void W(object value, bool addLineFeed = true)
        {
            WSTRING += UTILITY.PARSE<string>(value);
            if (addLineFeed == true) { WSTRING += "\n"; }
            if (WSTRING.Count() >= WMAX) { WFLUSH(); }
        }
        public static void WFLUSH()
        {
            Console.Write(WSTRING);
            WSTRING = "";
        }
    }
    public class UTILITY
    {
        public static T PARSE<T>(object value)
        {
            return (T)(Convert.ChangeType(value, typeof(T)));
        }
    }
}