def alternate(s):
    valid_substrings = []
    for first, second in combinations(set(s), 2):
        pattern = "[^" + first + second + "]"
        substring = re.sub(pattern, '', s)
        if not re.search(r'([a-z])\1', substring):
            valid_substrings.append(substring)
    return max([len(i) for i in valid_substrings]) if valid_substrings else 0

module Main where

import Data.List (group, nub, tails)
import Data.Maybe (mapMaybe)

onlyKeep :: (Eq a) => [a] -> [a] -> [a]
onlyKeep originals keepers = filter (`elem` keepers) originals

allPairs :: [a] -> [[a]]
allPairs xs = [[x, y] | (x : ys) <- tails xs, y <- ys]

lengthIfValid :: (Eq a) => [a] -> Maybe Int
lengthIfValid s =
    if and $ zipWith (/=) s (tail s)
        then Just $ length s
        else Nothing

solve :: (Eq a) => [a] -> Int
solve [] = 0
solve s =
    let us = nub s
        pairs = allPairs us
        ls = mapMaybe (lengthIfValid . onlyKeep s) pairs
     in maximum $ 0 : ls

main :: IO ()
main = getLine >> getLine >>= print . solve

int validSize(string s, char first, char second){
    string ans = "";
    for(int i = 0; i < s.size(); i++){
        if(s[i] == first || s[i] == second){
            if(ans.size() > 0 && ans[ans.size() - 1] == s[i]) return 0;
            else ans+=s[i];
        }
    }
    if(ans.size() < 2) return 0;
    return ans.size();
}

int alternate(string s) {
    int ans = 0;
    for(char i = 'a'; i < 'z'; i++){
        for(char j = i + 1; j <= 'z'; j++){
           int r = validSize(s, i, j);
           if(r > ans) ans = r;
        }
    }
    return ans;
}

import re
import itertools

def alternate(s):
    maxLen = 0
    unique_char = set(s)
    n = len(unique_char)
    pattern = re.compile(r'^(\w)(?!\1)(\w)(\1\2)*\1?$')
    
    if (n < 2):
        maxLen = 0
    else:
        for comb in itertools.combinations(unique_char, n-2):
            x = s
            
            for i in comb:
                x = x.replace(i, "")
            
            if (pattern.match(x)):
                maxLen = max(maxLen, len(x))
    
    return maxLen

length = int(input())
s = input()

result = alternate(s)

print(result)