Triangular numbers
题目描述
A. Triangular numbers
time limit per test
2 secondsmemory limit per test
256 megabytesinput
standard inputoutput
standard outputA 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
The first line contains the single number n (1≤n≤500) − the given integer.
Output
If the given integer is a triangular number output YES, otherwise output NO.
Examples
Input
1
Output
YES
Input
2
Output
NO
Input
3
Output
YES
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