<p>Given a string <code>word</code> to which you can insert letters "a", "b" or "c" anywhere and any number of times, return <em>the minimum number of letters that must be inserted so that <code>word</code> becomes <strong>valid</strong>.</em></p>
<p>A string is called <strong>valid </strong>if it can be formed by concatenating the string "abc" several times.</p>
<p> </p>
<p><strongclass="example">Example 1:</strong></p>
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<strong>Input:</strong> word = "b"
<strong>Output:</strong> 2
<strong>Explanation:</strong> Insert the letter "a" right before "b", and the letter "c" right next to "b" to obtain the valid string "<strong>a</strong>b<strong>c</strong>".
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<p><strongclass="example">Example 2:</strong></p>
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<strong>Input:</strong> word = "aaa"
<strong>Output:</strong> 6
<strong>Explanation:</strong> Insert letters "b" and "c" next to each "a" to obtain the valid string "a<strong>bc</strong>a<strong>bc</strong>a<strong>bc</strong>".
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<p><strongclass="example">Example 3:</strong></p>
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<strong>Input:</strong> word = "abc"
<strong>Output:</strong> 0
<strong>Explanation:</strong> word is already valid. No modifications are needed.
