CF977B Two-gram

Two-gram is an ordered pair (i.e. string of length two) of capital Latin letters. For example, "AZ", "AA", "ZA" — three distinct two-grams. You are given a string $ s $ consisting of $ n $ capital Latin letters. Your task is to find any two-gram contained in the given string as a substring (i.e. two consecutive characters of the string) maximal number of times. For example, for string $ s $ = "BBAABBBA" the answer is two-gram "BB", which contained in $ s $ three times. In other words, find any most frequent two-gram. Note that occurrences of the two-gram can overlap with each other.

The first line of the input contains integer number $ n $ ( $ 2 \le n \le 100 $ ) — the length of string $ s $ . The second line of the input contains the string $ s $ consisting of $ n $ capital Latin letters.

Print the only line containing exactly two capital Latin letters — any two-gram contained in the given string $ s $ as a substring (i.e. two consecutive characters of the string) maximal number of times.

In the first example "BA" is also valid answer. In the second example the only two-gram "ZZ" can be printed because it contained in the string "ZZZAA" two times.