华为OJ 初级：等差数列

import java.util.Scanner;

public class Main{
public static void main(String args[]) {
Scanner scanner = new Scanner(System.in);
int input = scanner.nextInt();
sum(input);
scanner.close();
}

public static void sum(int input) {
int num = -1;
int sum = 0;
for (int i = 0; i < input; i++) {  //当输入整数小于0时，循环终止，返回-1
num = num + 3;  //下一项的值
sum = sum + num;
}
System.out.println(sum);
}
}