#include <bits/stdc++.h>
using namespace std;

int main(){
    cin.tie(nullptr);
    ios::sync_with_stdio(false);

    long long n;
    cin >> n;

    long long ng = 0, ok = n;
    while((ok - ng) > 1){
        long long mid, t;
        mid = (ok + ng) / 2;
        t = mid * (1 + mid) / 2;
        if(t < n) ng = mid;
        else ok = mid;
    }

    if(ok * (1 + ok) / 2 == n) cout << "YES" << endl;
    else cout << "NO" << endl;

    return 0;
}