Statement

Valerio wants to compete in the IIOT Online Mirror, but first he has to create an account. Valerio usually uses the username S on the various websites he visits, but this username is very common, and sometimes he cannot use it because it was already taken by another user. In this case, Valerio will add a number at the end of his username. For example, if his username is "thescrasse" and the username is not available, then he will try to use "thescrasse1". Valerio will first try with the number 1, and if the new username is still not available, he will increment the number until he finds an available username. For example, if "thescrasse1" is not available, he will try "thescrasse2", then "thescrasse3", and so on. You know that the IIOT mirror already has N users, and you know the usernames U_i of all of them. Find the username T that Valerio will use.