import Control.Applicative
import Data.List

f 2 _ = "YES"
f n (x:xs) = if ((x2 - x1) == (x3 - x2)) && ((x2 - x1) /= 0) then f (n-1) xs else "NO"
	where
		x1 = x
		x2 = xs !! 0
		x3 = xs !! 1

main = do
  	n <- getLine
  	xs <- (map read . words) <$> getLine
  	putStrLn $ f (read n) (sort xs)