Triangular numbers

题意翻译

### 题面描述 给定一个数 $n$，问你是否存在一个整数 $i$，满足 $\frac{i\times (i+1)}{2}=n$。 若存在，输出`YES`,否则输出`NO`. ### 数据范围与约定 $1 \leq n \leq 500$

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.

输入输出格式

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

输入样例 #1

1

输出样例 #1

YES

输入样例 #2

2

输出样例 #2

NO

输入样例 #3

3

输出样例 #3

YES