fun main() {
    val sheepNumber = readLine()?.toInt()

    var i = 1
    sheepNumber?.let {
        var restSheepNumber = sheepNumber
        while (restSheepNumber > 0) {
            restSheepNumber -= i
            i++
        }

        if (restSheepNumber == 0) {
            println("YES")
        } else {
            println("NO")
        }
    }
}