Triangular numbers
TimeLimit:2000MS MemoryLimit:256MB
64-bit integer IO format:%I64d
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Problem 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
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.
SampleInput 1
1
SampleOutput 1
YES
SampleInput 2
2
SampleOutput 2
NO
SampleInput 3
3
SampleOutput 3
YES