SKK String

A string is called an SKK string if the number of K's it contains is exactly $2$ times the number of S's, and both S and K appear at least once.

An SKK string may contain letters other than S and K.

Given a string $S$ consisting only of uppercase letters, write a program that finds the longest SKK string among the substrings of $S$.

The first line gives a string $S$ consisting only of uppercase letters, with length between $1$ and $100,000$.

Print the length of the longest SKK string among the substrings of $S$. If no such string exists, print -1.

A new string formed by selecting consecutive characters from a string $S$ is called a substring of $S$.

For example, "appl", "ap", and "ple" are substrings of "apple", while "ppe" and "apl" are not substrings of "apple".