In this task, you need to count the number of times the given letter appears in the given sentence.

Example Input: Sentence: 'a man flying through the air on top of a skateboard'. Find frequency of the letter 'r'
Example Output: 3

Example Input: Sentence: 'a surfer wipes out on a huge wave'. Find frequency of the letter 'a'
Example Output: 3

Example Input: Sentence: 'a female skateboarder is in mid air doing a trick'. Find frequency of the letter 't'
Example Output:
2