Easy Subsequence Selection

Chef has always been fond of girls, but he could not make any girl his friend. In order to finally make one, he was asked to solve the following problem: You are given a string $S$ with length $N$. Choose an integer $K$ and two non-empty subsequences $A$ and $B$ of characters of this string, each with length $K$, such that: - $A = B$, i.e. for each valid $i$, the $i$-th character in $A$ is the