Contest1340/C.cpp

#include <bits/stdc++.h>
 
using namespace std;
int m;
 
bool Find(int x) {
    if (x * (x + 1) == m) {
        return true;
    }
    return false;
}
 
int main() {
    int n;
    while (cin >> n) {
        m = n <<= 1;
        bool flag = false;
        for (int i = 1; i < sqrt(m); i++) {
            if (Find(i)) {
                flag = true;
                break;
            }
        }
        if (flag) {
            cout << "YES" << endl;
        } else {
            cout << "NO" << endl;
        }
    }
    return 0;
}