题解 | #等差数列#抛砖引玉

#include<iostream>
using namespace std;

int main() {
    int n;
    cin >> n;
    int sum = 0;
    for (int i = 1; i <= n; i++) {
        sum += (3 * i - 1);
    }
    cout << sum << endl;
    return 0;
}