题解 | #等差数列#

等差数列求和公式

import java.util.*;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        while (sc.hasNext()) {
            int number = Integer.parseInt(sc.nextLine().trim());
            int a1= 2,d =3;
            System.out.println(number*a1+(number-1)*number*d/2);
        }
    }
}