#include <bits/stdc++.h>
using namespace std;
int main() {
  ios_base::sync_with_stdio(0), cin.tie(0), cout.tie(0);
  int n, c = 1, p = 2;
  cin >> n;
  while (c < n) {
    c += p;
    p++;
  }
  if (c == n)
    cout << "YES";
  else
    cout << "NO";
  return 0;
}
