题解 | #等差数列#

import java.util.*;

public class Main {

private int sum(int n) {
    return (1 + 3*n) * n / 2;
}

public Main() {
    Scanner in = new Scanner(System.in);
    while (in.hasNextInt()) {
        int n = in.nextInt();
        int res = sum(n);
        System.out.println(res);
    }

}

public static void main(String[] args) {
    Main solution = new Main();
}

}