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

Given a string word to which you can insert letters "a", "b" or "c" anywhere and any number of times, return the minimum number of letters that must be inserted so that word becomes valid.
A string is called valid if it can be formed by concatenating the string "abc" several times.
 
Please complete the following python code precisely:
```python
class Solution:
    def addMinimum(self, word: str) -> int:
```