## Algorithm

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 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.

Examples
input
Copy
`1`
output
Copy
`YES`
input
Copy
`2`
output
Copy
`NO`
input
Copy
`3`
output
Copy
`YES`

## Code Examples

### #1 Code Example with C++ Programming

```Code - C++ Programming```

``````#include <iostream>

using namespace std;

int main() {
int n, c = 1;
cin >> n;

for(int i = 1; c <= n; i++, c += i)
if(c == n) {
cout << "YES" << endl;
return 0;
}

cout << "NO" << endl;

return 0;
}``````
Copy The Code &

Input

cmd
1

Output

cmd
YES