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

Example Input: Sentence: 'a fluffy white car looking in a mirror'. Find frequency of the letter 'k'
Example Output: 1

Example Input: Sentence: 'a red building with a crowd of people outside it near the water'. Find frequency of the letter 'e'
Example Output: 7

Example Input: Sentence: 'a red double decker bus driving down a street'. Find frequency of the letter 'l'
Example Output:
1