|Codeforces Beta Round #44 (Div. 2)|
A triangular number is the number of dots in an equilateral triangle uniformly filled with dots. For example, three dots can be arranged in a triangle; thus three is a triangular number. The n-th triangular number is the number of dots in a triangle with n dots on a side. . You can learn more about these numbers from Wikipedia (http://en.wikipedia.org/wiki/Triangular_number).
Your task is to find out if a given integer is a triangular number.
The first line contains the single number n (1 ≤ n ≤ 500) — the given integer.
If the given integer is a triangular number output YES, otherwise output NO.