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

Example Input: Sentence: 'a picture of a machine built next to a street'. Find frequency of the letter 'x'
Example Output: 1

Example Input: Sentence: 'man in a sleeping bag on the floor using a laptop computer'. Find frequency of the letter 'i'
Example Output: 3

Example Input: Sentence: 'an older man walking with a cane crossing the street'. Find frequency of the letter 'a'
Example Output:
5