### Minimum Additions to Make Valid String

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.

 

**Example 1:**

```
Input: word = "b"
Output: 2
Explanation: Insert the letter "a" right before "b", and the letter "c" right next to "a" to obtain the valid string "abc".
```

**Example 2:**

```
Input: word = "aaa"
Output: 6
Explanation: Insert letters "b" and "c" next to each "a" to obtain the valid string "abcabcabc".
```

**Example 3:**

```
Input: word = "abc"
Output: 0
Explanation: word is already valid. No modifications are needed. 
```

 

**Constraints:**

- `1 <= word.length <= 50`
- `word` consists of letters "a", "b" and "c" only. 

**Function definition**

```
class Solution:
    def addMinimum(self, word: str) -> int:
        
```

