Exercise: Grade Student |2|
~~~~~~~~~~~~~~~~~~~~~~~~~~~

You are a teacher who is reading exam papers. Since it can be tiring to
read and grade hundreds of papers, you decide to write a function that
does the job for you.

Write a function named *grade_student* which gets two *strings* as the
file names. The first file contains the answer key, where each line
holds a record of question’s number, answer and grade separated by a
blank character. The second file contains a student’s answers, where
each line holds a record of question’s number and student’s answer. This
function should return an *integer* which is the total grade student
gets.

Note that your students answer all the questions. That means both files
always have the same number of lines.

.. container:: sampleio

   Sample I/O:

.. |2| image:: ../../figures/difficulty_two.png
   :class: difficulty

.. code:: default

   Sample function call:  
       grade_student("answer_key.txt", "student.txt")  

   Content of the file "answer_key.txt":  
       1 A 5
       2 C 15
       3 A 10
       4 B 5
       5 D 5
       6 C 15
       7 C 15
       8 A 10
       9 D 15
       10 C 5

   Content of the file "student.txt":  
       1 A
       2 C
       3 D
       4 A
       5 C
       6 C
       7 D
       8 A
       9 D
       10 C


   Return value:
       65