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How many strings of length 8 formed using letters from $\{a,b,c,d\}$ contain exactly one pair of adjacent letters that are the same?

Can any kind soul give some hint?

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Hint:

How many strings of length $8$ using those letters have the same first and second letters and have no other adjacent letters that are the same?

How many strings of length $8$ using those letters have the same second and third letters and have no other adjacent letters that are the same?

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