K-th Character in String Game I

A game starting with the string "a". In each round, append the next letter of the alphabet after each existing character. Find the k-th character (1-indexed) after enough rounds. The character is determined by the number of set bits in the binary representation of k - 1.

Examples
Input: 1
Output: "a"
Hints

K-th Character in String Game I

A game starting with the string `"a"`. In each round, append the next letter of the alphabet after each existing character. Find the k-th character (1-indexed) after enough rounds. The character is determined by the number of set bits in the binary representation of `k - 1`.