There is a game called Jisho in Japan. In that, players are given an integer $$$alpha$$$ and a string $$$beta$$$ to play with. The players have to choose some positions (indexes) from the string such that at least 1 position from the chosen set is present in any continuous substring of length $$$alpha$$$.

The players then have to form the smallest lexicographic string with the help of the letters corresponding to those indices (they can rearrange the letters to form the smallest lexicographic string).

Adi wants to win the game, therefore, he comes to you for help. Help Adi by returning the smallest lexicographic string possible with these conditions.

You can learn more about lexicographic strings here.