题解 | #等差数列#

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        while (sc.hasNext()) {
            int n = sc.nextInt();
            int start = 2;
            int sum = 0;
            for (int i = 0; i < n; i++) {
                sum += start;
                start += 3;
            }
            System.out.println(sum);
        }
    }
}