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1{-# OPTIONS_GHC -O2 -XNoMonomorphismRestriction #-}
2{-# LANGUAGE BangPatterns #-}
3{-(c) gorlum0 [at] gmail.com-}
4import Control.Monad (forM_)
5
6triangular = takeWhile (<=500) $ scanl1 (+) [1..]
7
8main = do
9    ls <- lines `fmap` getContents
10    let xs = map read ls
11    forM_ [x `elem` triangular | x <- xs] $
12        putStrLn . (\p -> if p then "YES" else "NO")