#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <math.h>
#include <iostream>
#include <algorithm>
//#include <vector>

using namespace std;

#define MAX(x,y) (x>y?x:y)
#define MIN(x,y) (x<y?x:y)
#define ABS(x) (x>0?x:-(x))
#define INF 1000000000

void swap(int* a, int* b)
{
	int tmp;
	tmp = *a;
	*a = *b;
	*b = tmp;
}

void swap2(double* a, double* b)
{
	double tmp;
	tmp = *a;
	*a = *b;
	*b = tmp;
}

int gcd(int a, int b)
{
	if (a < b)
		swap(&a, &b);

	if (a % b == 0)
		return b;
	else
		return gcd(b, a % b);
}

int lcm(int a, int b)
{
	return a * b / gcd(a, b);
}

int factorial(int n)
{
	int a=1;

	if (n == 0)
		return 1;

	for (int i = 1; i <= n; i++)
		a *= i;

	return a;
}

int Con(int a, int b)
{
	return factorial(a) / (factorial(b) * factorial(a - b));
	
}

int a[20] = { 0 };

//int binary_search(int key)  
//{
//	int low, high, middle;
//
//	low = 0;
//	high = 20 - 1;
//
//
//
//	while (low <= high)
//	{
//		middle = (low + high) / 2;
//		if (key == a[middle])
//			return middle;
//		else if (key < a[middle])
//			high = middle - 1;
//		else if (key > a[middle])
//			low = middle + 1;
//	}
//	return INF;
//}

int binary_search(long long int key)
{
	long long int low = 0, high = 2000000000, middle;
	long long int p;

	while (low <= high)
	{
		middle = (low + high) / 2;
		//printf("%d    \n", middle);

		p = (middle * (middle + 1));

		if (p == key*2)
			return 1;
		else if (p >key*2)
			high = middle - 1;
		else
			low = middle + 1;
	}
	return 0;

}

int main()
{
	//for (int i = 0; i < 20; i++)
	//	a[i] = rand() % 50;
	//
	//sort(a, a + 20);


	//for (int i = 0; i < 20; i++)
	//	printf("%d\n", a[i]);

	//for (int i = 0; i < 20; i++)
	//{
	//	printf("    %d\n",binary_search(i));
	//}
	
	long long int n;

	scanf("%lld", &n);

	if (binary_search(n) == 1)
		printf("YES");
	else
		printf("NO");

}