For an integer n, we call k>=2 a good base of n, if all digits of n base k are 1. Now given a string representing n, you should return the smallest good base of n in string format. Example 1: Input: "13" Output: "3" Explanation: 13 base 3 is 111. Example 2: Input: "4681" Output: "8" Explanation: 4681 base 8 is 11111. Example 3: Input: "1000000000000000000" Output: "999999999999999999" Explanation: 1000000000000000000 base 999999999999999999 is 11. Note: The range of n is [3, 10^18]. The string representing n is always valid and will not have leading zeros.