#include<iostream>
#include<math.h>

using namespace std;

// N=n(n+1)/2
// 2N=n(n+1)
// -->(int)√N * (int)√N +1 ?
int main(){
    long long n;

    cin>>n;
    if((long long)sqrt(2*n)*((long long)sqrt(2*n)+1)==2*n) cout<<"YES"<<endl;
    else cout<<"NO"<<endl;
}