{-# LANGUAGE BangPatterns #-}
{-# LANGUAGE BinaryLiterals #-}
{-# LANGUAGE MultiWayIf #-}
{-# LANGUAGE NumDecimals #-}
{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE TupleSections #-}
{-# LANGUAGE FlexibleContexts #-}

{-# OPTIONS_GHC -O2 #-}
{-# OPTIONS_GHC -Wno-tabs #-}

import Control.Applicative
import Control.Arrow
import Control.Monad
import Control.Monad.ST

import Data.Char
import Data.List
import Data.Maybe

import qualified Data.ByteString.Char8 as B

import Data.Array
import Data.Array.ST.Safe
import Data.STRef

import Debug.Trace
import Text.Printf

readInt = readLn :: IO Int
readInteger = readLn :: IO Integer
readInts = map ( fst . fromJust . B.readInt ) . B.words <$> B.getLine
readIntegers = map ( fst . fromJust . B.readInteger ) . B.words <$> B.getLine

which a b f = if f then a else b
mp [ a, b ] = ( a, b )

modifyArray a i f = writeArray a i =<< f <$> readArray a i

printList :: Show a => [a] -> IO ()
printList = putStrLn . intercalate " " . map show

modulo = 998244353

main = do
	[ m, n ] <- readInts
	xs <- ( -1 : ) . ( ++ [ fromIntegral m ] ) . map pred <$> readIntegers
	print $ ( `mod` modulo ) $ sum $ map ( sqsum . pred ) $ zipWith (-) ( tail xs ) xs

sqsum n = n * ( n + 1 ) * ( 2 * n + 1 ) `div` 6 `mod` modulo