#include <bits/stdc++.h>   using namespace std;   int main() {     int n, cnt = 0;     cin >> n;     for (int i = 1; i <= n; i++) {       int ans = (i * (i + 1)) / 2;       if (ans == n) {         cnt = 1;         break;       }     }     if (cnt == 1)       cout <<  YES  << endl;     else       cout <<  NO  << endl;   }   