import Data.List(sort)

main :: IO ()
main = getContents >>= putStrLn . compute . sort . tail . map readInt . words

readInt :: String -> Int
readInt = read

compute :: [Int] -> String
compute (x1:x2:xs)
  | x2-x1 == 0 = "NO"
  | otherwise = checkList xs (x2-x1) x2

checkList :: [Int] -> Int -> Int -> String
checkList [] _ _ = "YES"
checkList (x:xs) diff lst
  | lst == x - diff = checkList xs diff x
  | otherwise = "NO"