import Control.Applicative ((<$>))
import qualified Data.IntSet as S
import Data.List (sort)

-- 与えられた数列の階差数列を求める関数
-- WriterによるPython解と同様の考え方による
diff :: Num a => [a] -> [a]
diff xs = zipWith (-) (tail xs) (init xs)

-- 与えられた数列が二条件を同時に満たすかを判定する関数
isKamo :: [S.Key] -> Bool
isKamo = f . S.fromList . diff . sort
    where f s = S.size s == 1 && S.notMember 0 s

main :: IO ()
main = do
    -- Nは使わないので読み捨てる
    _ <- getLine
    xs <- fmap read . words <$> getLine :: IO [Int]
    putStrLn $ if isKamo xs then "YES" else "NO"