#include <iostream>
#include <iomanip>
#include <cstring>
#include <algorithm> 
#include <math.h>
#include <queue>
#include <functional>
#include <map>
#include <vector>
#include <string>
using namespace std;

typedef long long ll;
typedef pair<int, int> P;

int dx[] = { 1, 0, -1, 0 };
int dy[] = { 0, 1, 0, -1 };

const ll MOD = 1000000007;
const ll INF = 100000;


ll n;
int main(void){
    cin >> n;
    ll s = (ll)sqrt(2 * n);
    if (s*(s + 1) == 2 * n) cout << "YES" << endl;
    else cout << "NO" << endl;

}