import Data.List(sort)

main :: IO ()
main = getContents >>= putStrLn . checkList [] . sort . tail . map readInt . words

readInt :: String -> Int
readInt = read

checkList :: [Int] -> [Int] -> String
checkList (x:xs) [_] = if x /= 0 && all (x==) xs then "YES" else "NO"
checkList cs (x:y:xs) = checkList (y-x:cs) (y:xs)