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: 'a plastic container next to a toilet on a wooden box'. Find frequency of the letter 'i'
Example Output: 3

Example Input: Sentence: 'apples in a wooden bowl with wooden spoons'. Find frequency of the letter 'a'
Example Output:
2