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