A_Triangular_numbers.cpp

#include<bits/stdc++.h>

using namespace std;


bool isTriangular(int num)
{
    if (num < 0)
        return false;
 
    int c = (-2 * num);
    int b = 1, a = 1;
    int d = (b * b) - (4 * a * c);
 
    if (d < 0)
        return false;
 
    
    float root1 = ( -b + sqrt(d)) / (2 * a);
    float root2 = ( -b - sqrt(d)) / (2 * a);
 
   
    if (root1 > 0 && floor(root1) == root1)
        return true;
 
    
    if (root2 > 0 && floor(root2) == root2)
        return true;
 
    return false;
}
 

int main()
{
    int num ;
    cin>>num;
    if (isTriangular(num))
        cout << "YES"<<endl;
    else
        cout << "NO"<<endl;
 
    return 0;
}