Word Tower Construction

You are given a list of N unique words. A 'Word Tower' is a sequence of K words from the list where each word (except the first) must begin with the last two letters of the previous word. Each word in the tower must be unique. Calculate the total number of unique Word Towers of length exactly K that can be built. Input: First line: K and N. Following N lines: One word per line. Output: Total number of valid sequences. Example: Input: 3 4 apple lemon onion only Output: 1 Explanation: The only sequence of length 3 is apple -> lemon -> onion (le-le, on-on).