A. Triangular numbers

time limit per test

2 seconds

memory limit per test

256 megabytes

input

standard input

output

standard output

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 ndots 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.

Sample test(s)

input

1

output

YES

input

2

output

NO

input

3

output

YES

解题说明：关于什么是三角数请看这里：http://en.wikipedia.org/wiki/Triangular_number 判断一个数是不是三角数就只需要判断该数是否等于n(n+1)/2，由于输入不超过500，那么n值不超过33，暴力即可。

#include <iostream>#include <cstdio>#include <cstdlib>#include <cmath>#include <cstring>#include <string>#include<set>#include <algorithm>using namespace std;int main(){int n,c=1,f=0;scanf("%d",&n);    for(;c<33;c++){if(n==c*(c+1)/2){f=1;}}if(f==1){printf("YES\n");}else{printf("NO\n");}}