public static void Main()
string s1 = "burgas", s2=" e nai-dobriqt", s3=" grad za jiveene";
Console.WriteLine(s1.Length);
string s4 = s1.ToUpper();
string jechev = string.Concat(s1, s2, s3);
Console.WriteLine(jechev);
Console.WriteLine(jechev.IndexOf("u"));