Given a string word containing only the letters "a", "b", and "c", return the minimum number of characters that must be inserted into word to make it valid.

A string is considered valid if it can be formed by concatenating the string "abc" one or more times.

Constraints:

• $1 \leq$ word.length $\leq 50$

• word consists of letters "a", "b", and "c" only.

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