求s=a+aa+aaa+aaaa+aa...a的值

求s=a+aa+aaa+aaaa+aa...a的值，其中a是一个数字。
例如2+22+222+2222+22222(此时共有5个数相加)，几个数相加有键盘控制。

import java.util.*;

public class lianxi05{
    public static void main(String[] args){
        Scanner sc = new Scanner(System.in);
        System.out.println("输入数字a的值：");
        int a = sc.nextInt();
        System.out.println("输入相加项数：");
        int n = sc.nextInt();
        int b = 0, sum = 0;
        
        /* 方法1：b = 2; b = 20; b = 22; b = 220 ....
        for(int i = 1; i <= n; i++){
            b = b + a;
            sum += b;
            b *= 10;
        }
        
        */
        /* 方法2：b = 2, a = 20, b = 22, a = 200, b = 222, a = 2000, b = 2222...
        */
        for(int i = 1; i <= n; i++){
            b = b + a;
            sum += b;
            a *= 10;
        }
        
        System.out.println("计算结果为：" + sum);
        
    }
}