#include <iostream>
#include <cmath>

using namespace std;
using LL = long long;
using ULL = unsigned long long;

int main() {
    ULL n;
    cin >> n;

    ULL i = sqrt(n * 2);

    if (n * (n + 1) / 2 == n) {
        cout << "YES" << endl;
    } else {
        cout << "NO" << endl;
    }
}