#danglam228

#include <bits/stdc++.h>
using namespace std;
int n;
int main() {
    
    cin >> n;
    int d1 = 1,d2 = 0; 
    while (d2 < n) {
        d2 += d1; 
        if (d2 == n) {
            cout << "Yes." << endl;
            return 0;
        }
        ++d1;
    }
    cout << "No." << endl;
    return 0;
}

`color(#5C73B9ff)(H)color(#6B68C0ff)(o)color(#795DC6ff)(r)color(#8852CDff)(i)color(#9646D4ff)(z)color(#A53BDAff)(o)color(#B330E1ff)(n)`

#include <iostream>
using namespace std;

int main() {
    int n, i = 1, j = 0;
    cin >> n;
    while (j < n) {
        j += i++;
        if (j == n) {
            cout << "Yes." << '\n';
            return 0;
        }
    }
    cout << "No." << '\n';
    return 0;
}