import math
n=int(input())
if pow(int(math.sqrt(1+(8*n))),2)==1+(8*n):
print("YES")
else:
print("NO")#include <bits/stdc++.h>
using namespace std;
int main() {
int n;
cin >> n;
int i = 1;
while (i * (i + 1) < 2 * n) {
i++;
}
if (i * (i + 1) == 2 * n) {
cout << "YES";
} else {
cout << "NO";
}
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