using System;
using System.IO;
using System.Linq;
using System.Collections.Generic;
using System.Text;

public class Program
{

    public void Proc()
    {
        int[] inpt = Reader.ReadLine().Split(' ').Select(a => int.Parse(a)).ToArray();

        long num1 = inpt[1] - inpt[0];
        long num2 = inpt[2] - inpt[1];

        long ans = 0;
        if(num1 == num2) {
            ans = inpt[2] + num1;
        }
        if(num2%num1==0) {
            ans = inpt[2] + (num2 * (num2 / num1));
        }
        if(num1%num2 ==0) {
            ans = inpt[2] + (num2 / (num1 / num2));
        }
        Console.WriteLine(ans);


    }

    public class Reader
	{
		private static StringReader sr;
		public static bool IsDebug = false;
		public static string ReadLine()
		{
			if (IsDebug)
			{
				if (sr == null)
				{
					sr = new StringReader(InputText.Trim());
				}
				return sr.ReadLine();
			}
			else
			{
				return Console.ReadLine();
			}
		}
		private static string InputText = @"




1 2 100000




";
	}

	public static void Main(string[] args)
	{
#if DEBUG
		Reader.IsDebug = true;
#endif
		Program prg = new Program();
		prg.Proc();
	}
}