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.

二元组（two-gram）是指一个有序对（即长度为 2 的字符串），由大写拉丁字母组成。例如，“AZ”、“AA”、“ZA” 是三个不同的二元组。

给定一个由 $n$ 个大写拉丁字母组成的字符串 $s$。你的任务是在该字符串中找出任意一个作为子串（即字符串中两个连续的字符）出现次数最多的二元组。例如，对于字符串 $s = \text{“BBAABBBA”}$，答案是二元组 “BB”，它在 $s$ 中出现了三次。换言之，请找出任意一个出现频率最高的二元组。

注意：二元组的出现位置可以相互重叠。

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.

输入的第一行包含一个整数 $n$（$2 \le n \le 100$）—— 字符串 $s$ 的长度。
输入的第二行包含字符串 $s$，由 $n$ 个大写拉丁字母组成。

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.

输出唯一一行，包含恰好两个大写拉丁字母——即字符串 $s$ 中作为子串（即字符串中两个连续字符）出现次数最多的任意一个二元组。

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.

在第一个例子中，“BA”也是一个有效的答案。

在第二个例子中，唯一的二元组“ZZ”可以被输出，因为它在字符串“ZZZAA”中出现了两次。