#include <bits/stdc++.h>
using namespace std;
int main() {
  int n;
  cin >> n;
  int sum = 0;
  bool ok = false;
  for (int i = 1; i <= n; i++) {
    sum = i + sum;
    if (n == 1 || n == sum) {
      cout << "YES" << endl;
      break;
    }
    if (sum > n) {
      ok = true;
      break;
    }
  }
  if (ok) cout << "NO" << endl;
  return 0;
}
