Please solve the programming task below using a self-contained code snippet in a markdown code block.

Given two strings a and b, return the minimum number of times you should repeat string a so that string b is a substring of it. If it is impossible for b​​​​​​ to be a substring of a after repeating it, return -1.
Notice: string "abc" repeated 0 times is "", repeated 1 time is "abc" and repeated 2 times is "abcabc".
 
Please complete the following python code precisely:
```python
class Solution:
    def repeatedStringMatch(self, a: str, b: str) -> int:
```