#include <bits/stdc++.h>
using namespace std;
#define _for(i,a,b) for (int i=(a); i<(b); i++)
#define __for(i,a,b) for (int i=(a); i<=(b); i++)
#define endl "\n"
int main(){
    ios::sync_with_stdio(false);
    cin.tie(NULL);
    cout.tie(NULL);
    int n;
    cin >> n;
    n *= 2;
    if ( (int)sqrt(n) * (int(sqrt(n)) + 1) == n ) cout << "YES";
    else cout << "NO";
	return 0;
}