Prompt User To Enter Integer From 1 To 15 And Displays A Pyramid

The exercise sounds like this : Write a program that prompts the user to enter an integer from 1 to 15 and displays a pyramid , as shown in the following sample run (my code displays correctly the first 9 lines):

1
2 1 2
3 2 1 2 3
4 3 2 1 2 3 4
5 4 3 2 1 2 3 4 5
6 5 4 3 2 1 2 3 4 5 6
7 6 5 4 3 2 1 2 3 4 5 6 7

The problem is when i input a number greater then 9 as it requires 2 spaces . I m pritty sure i solved it incorrectly or at lost not optimal as i m using a string that decreases on each line to create the pyramid effect.

import java.util.*;
public class C5_17 {
public static void main(String[] args){
Scanner input = new Scanner(System.in);
System.out.println("Enter the number of lines: ");

[Code] ....