CF47A.Triangular numbers

普及/提高-

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题目描述

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.

输入格式

The first line contains the single number $n$ ( $1<=n<=500$ ) — the given integer.

输出格式

If the given integer is a triangular number output YES, otherwise output NO.

输入输出样例

输入#1
```
1
```
输出#1
```
YES
```
输入#2
```
2
```
输出#2
```
NO
```
输入#3
```
3
```
输出#3
```
YES
```

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