In this challenge, you will be given a string. You must remove characters until the string is made up of any two alternating characters. When you choose a character to remove, all instances of that character must be removed. Your goal is to create the longest string possible that contains just two alternating letters.

Sample Input
10
beabeefeab

Sample Output
5
Explanation

The characters present in are a, b, e, and f. This means that must consist of two of those characters and we must delete two others. Our choices for characters to leave are [a,b], [a,e], [a, f], [b, e], [b, f] and [e, f].

If we delete e and f, the resulting string is babab. This is a valid as there are only two distinct characters (a and b), and they are alternating within the string.

If we delete a and f, the resulting string is bebeeeb. This is not a valid string because there are consecutive e's present. Removing them would leave consecutive b's, so this fails to produce a valid string .

Other cases are solved similarly.

babab is the longest string we can create.