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Time limit 2000/4000/4000/4000 ms. Memory limit 65000/65000/65000/65000 Kb. 
  
 Triangular Numbers 
Shqip 
Square Numbers are the numbers which can be represented as a square; like 4, 16  or 25.  
   
Triangular Numbers are similar to square numbers but they form a triangle. 
   
Write a program that is going to accept an integer number and decide if the number is a triangular number or not.
 
Input specification   
There will be just one integer number n where 1 ≤ n ≤ 1 billion.
 
 
Output specification   
Print out "YES" if the given number is a triangular number. "NO", otherwise.  
 Sample Input:    
5  
 
 Sample Output:    
NO
 
 Sample Input:    
21  
 
 Sample Output:    
YES
 
 Для отправки решений необходимо выполнить вход.
  
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