A8063. Triangular numbers

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.