Walking along, Tintin overhead the two island gate guards, Thomas and Thompson chatting amongst themselves.
Thomson : He told us we have consecutive letters on our papers
Thompson : Yeah he told me too.
Thomson : I don't know what your letter is.
Thompson : Neither do I know your letter.
Thomson : Now I know. Tintin listened this conversation and wondered what the two letters could be. When he was about to leave the island, Thomson stopped him and told him to say the password, which were the two letters. Maximum of how many attempts does Tintin need to make to have the correct pair of letters, and thus pass successfully?