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Triangular numbers

题面翻译

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.

样例 #1

样例输入 #1

1

样例输出 #1

YES

样例 #2

样例输入 #2

2

样例输出 #2

NO

样例 #3

样例输入 #3

3

样例输出 #3

YES