CF47A Triangular numbers
Description
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.
Input Format
The first line contains the single number $ n $ ( $ 1
Output Format
If the given integer is a triangular number output YES, otherwise output NO.