Now you are asked for help by the organizers of the Winter Olympic Games 2014,
which, as you know, will be held in Yekaterinozavodsk. And although there are
still 6 years ahead, the first sport facility is already put into operation.
It is the track for the skiing race.
It was decided to construct a huge screen near the tribunes, which would show
the name of a current leader during the race. It is known, that only the first
N letters of English alphabet are used to write the names of the sportsmen, and
each of the N letters should appear in each name exactly once. Technically,
the screen looks like a word with letters from the English alphabet; each of these letters can either shine or stay dark. The organizers call the word
“universal” if the screen based on it can display any possible name of a
sportsman (possibly with some holes between neighboring letters). The governor
proposed some word for the screen. You should tell whether this word is
The first line contains an integer N (1 ≤ N ≤ 26). The second line contains
a word to check, consisting of lowercase English letters. The word can contain
only first N letters of the alphabet. The length of the word doesn't exceed 300.
Output “YES”, if the word, proposed by governor, is universal and “NO” otherwise.
Problem Author: Ivan Burmistrov (prepared by Alex Samsonov)
Problem Source: Ural SU Contest. Petrozavodsk Winter Session, January 2008