using System;

using System.Collections.Generic;

public class Program

{

    public static void Main()

    {

        int[] n = [1, 9, 9, 9, 1];

        Console.WriteLine(GcdOfStrings("TAUXXTAUXXTAUXXTAUXXTAUXX","TAUXXTAUXXTAUXXTAUXXTAUXXTAUXXTAUXXTAUXXTAUXX"));

    }

    public static string GcdOfStrings(string str1, string str2) {

​

        if(str1.Length < str2.Length){

            string temp = str1;

            str1 = str2;

            str2 = temp;

        }

        while(str2 != ""){

            if(str1.Substring(0,str2.Length) == str2){

                str1 = str1.Substring(str2.Length);

                if(str1.Substring(str2.Length).Length < str2.Length){

                string temp = str2;

                str2 = str1;

                str1 = temp;

                }

            }

            else return "";

        }

        return str1;     

            }   

}

​