#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
const int MOD = 1000000007;
const ll INF = 2000000000;

#define REP(i,n) for((i)=0;(i)<(int)(n);(i)++)

ll n;

ll func(ll x){
    return x * (x + 1) / 2;
}

bool C(ll k){
    return func(k) >= n;
}

int main(){
    cin >> n;

    ll ng = 0;
    ll ok = INF;
    
    while(abs(ok - ng) > 1){
        ll mid = (ok + ng) / 2;
        if (C(mid)) ok = mid;
        else ng = mid;
    }

    if (func(ok) == n) cout << "YES" << endl;
    else cout << "NO" << endl;

    return 0;
}