let rec sqrt n guess =
    let curGuess =
        match guess with
        | None    -> n / 2m
        | Some(x) -> x
    let res = n / curGuess
    let ave = (curGuess + res) / 2m
    if ave = curGuess then ave
    else sqrt n (Some(ave))

let isInt (n:decimal) =
    let str = n.ToString().Split('.')
    str.Length > 1 &&
    str.[1].ToCharArray() |> Array.forall(fun x -> x ='0')

let isValid n =
    let f n = ((sqrt (8m * n + 1m) None )-1m)/2m
    f n |> isInt
    
let n = stdin.ReadLine() |> decimal

n
|> isValid
|> function | true -> "YES" | _ -> "NO"
|> printfn "%s"