数列求和 [输入样例] 5 2 [输出样例] 2 22 222 2222 22222 24690 [输入样例] 10 8 [输出样例] 8 88 888 8888 88888 888888 8888888 88888888 888888888 8888888888 9876543200 [输入样例] 3 7 [输出样例] 7 77 777 861