a+aa+...+aaa..aa表达式输出

输入两个正整数a和n,输出a+aa+aaa+…+aaa…aaa

import java.util.Scanner;
public class Main{
    public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int a = sc.nextInt();
		int n = sc.nextInt();
		int num = 0;
		int sum = 0;
		String str = "";

		if (a > 0) {
			for (int i = 0; i < n; i++) {
				num = num * 10 + a;
				if (i < n-1) {
					str = str + num + "+";
				} else {
					str = str + num;
				}
				sum = sum + num;
			}
			System.out.println(str + "=" + sum);
		}

	}

}