#include <bits/stdc++.h>
using namespace std;
const long long MXN = 1e5 + 1;
const long long MNN = 1e3 + 1;
const long long MOD = 1e9 + 7;
const long long INF = 1e18;
const long double EPS = 1e-9;
const long double PI = 3.1415926536;
const string TIME = "rovno_ushten_push_ketti";
long long n, road_to_respa_bronze;
long long dp[MXN];
string ans[5] = {
    "NO",
    "YES",
};
int main() {
  ios_base::sync_with_stdio(0);
  cin >> n;
  for (long long i = 1; i <= 100; i++) {
    dp[road_to_respa_bronze + i] = 1;
    road_to_respa_bronze += i;
  }
  cout << ans[dp[n]];
  return 0;
}
