Defining a Figure of Merit

Consider a string-matching figure of merit. That is, it tells you how close to a given string another string is. Each matching letter in the same spot is worth one point. Only letters need be considered.

For instance, if the secret string reads 'BLACKBEARD', then 'BEACKBEARD' is worth 9 points, 'WHITEBEARD' is worth 5 points, 'BEARDBLACK' is worth 4 points, and 'CALICOJACK' is worth 1 point.

Compose a function pirate which accepts a string of characters guess and returns the number of characters which match the secret string 'BLACKBEARD'. It should be case-insensitive; that is, you should convert input to upper-case letters. It should return zero for strings which are not ten characters in length.

Your submission should include a function pirate( guess ) which returns a float or int representing the number of matching characters.(You should provide the secret string 'BLACKBEARD' inside the function, not outside of it.)

python 3

how do I code this