#!/bin/python3

import math
import os
import random
import re
import sys

#
# Complete the 'alternate' function below.
#
# The function is expected to return an INTEGER.
# The function accepts STRING s as parameter.
#
def consecutive_chars(s):
    for i in range(len(s) - 1):
        if s[i] == s[i + 1]:
            return 0
    return len(s)
    
def alternate(s):
    # Write your code here
    tmp_list = list(set(s))
    if len(tmp_list) < 2:
        return 0
    tmp_list = sorted(tmp_list)
    length_list = []
    for i in range(len(tmp_list) - 1):
        for j in range(i+1, len(tmp_list)):
            new_s = [char for char in s if char == tmp_list[i] or char == tmp_list[j]]
            length_list.append(consecutive_chars(new_s))
    return max(length_list)
        

if __name__ == '__main__':
    fptr = open(os.environ['OUTPUT_PATH'], 'w')

    l = int(input().strip())

    s = input()

    result = alternate(s)

    fptr.write(str(result) + '\n')

    fptr.close()