#include <bits/stdc++.h>   using namespace std;   long long int a, b;   bool temp = false;   int main() {     cin >> a;     for (b = 1; b <= 500; b++)       if ((b * (b + 1)) / 2 == a) {         temp = true;         break;       }     if (temp)       cout <<  YES ;     else       cout <<  NO ;     return 0;   }   