You are to generate codes consisting of four letters, using the letters A, B, C and D. The rule is that, in each code, at least two letters that stand side by side should be the same. Applying this rule, how many different codes can you generate?
Examples: AAAA, CBBD, CCAA, DBBB.
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This question was prepared by Emrehan Halıcı, president of the Turkey Intelligence Foundation, for Bilkent IEEE.