Solve the programming task below in a Python markdown code block.
Given a string, 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.  Determine the longest string possible that contains just two alternating letters.

Example    

$s=\text{'abaacdabd'}$   

Delete a, to leave bcdbd.  Now, remove the character c to leave the valid string bdbd with a length of 4. Removing either b or d at any point would not result in a valid string.  Return $4$.   

Given a string $s$, convert it to the longest possible string $t$ made up only of alternating characters.  Return the length of string $t$.  If no string $t$ can be formed, return $0$.

Function Description

Complete the alternate function in the editor below.  

alternate has the following parameter(s):  

string s: a string   

Returns. 

int: the length of the longest valid string, or $0$ if there are none   

Input Format

The first line contains a single integer that denotes the length of $s$. 

The second line contains string $s$.

Constraints

$1\leq\text{length of}\text{ s}\leq1000$
$s[i]\in\operatorname{ascii[a-z]}$ 

Sample Input
STDIN       Function
-----       --------
10          length of s = 10
beabeefeab  s = 'beabeefeab'

Sample Output
5

Explanation

The characters present in $s$ are a, b, e, and f. This means that $t$ 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 $t$ 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 $t$ because there are consecutive e's present.  Removing them would leave consecutive b's, so this fails to produce a valid string $t$.

Other cases are solved similarly.

babab is the longest string we can create.