按题意模拟即可。

AC code：

#include <bits/stdc++.h>
using namespace std;

int main() {
    long long n;
    cin >> n;
    vector<long long> valid;
    for (long long x = 1; 2 * x <= n; x++) {
        valid.push_back(x);
    }
    if (valid.empty()) {
        cout << "NO!" << endl;
    } else {
        cout << valid.size() << endl;
        for (long long x : valid) {
            cout << x << " ";
        }
        cout << endl;
    }
    return 0;
}