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

Example Input: Sentence: 'person wearing all white leaned up against a wall with a yellow sign'. Find frequency of the letter 'a'
Example Output: 8

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: 'a man leans over a desk in a hotel room'. Find frequency of the letter 'v'
Example Output:
1