SP8750 WORD - Wordplay

Ivana made up a long word of N letters. Then she wrote down all K-letter-substrings of that word. For example, if the original word is BANANA and K=3, Ivana writes down the words BAN, ANA, NAN, ANA. The number of these words is, obviously, N-K+1. Ivana sorted these words in lexicographic order (in the given example, that would be ANA, ANA, BAN, NAN). But the sad thing happened: Ivana forgot the original word! Your task is to reconstruct it. A unique solution will exist in all of the test data. Constraints: 3

\[integers N, K\] \[N-K+1 words in lexicographic order, each consisting of capital English letters\]

\[the required word\]