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

Example Input: Sentence: 'a bird outside standing on a table looking up'. Find frequency of the letter 'b'
Example Output: 2

Example Input: Sentence: 'a police officer who is riding on a horse'. Find frequency of the letter 'p'
Example Output: 1

Example Input: Sentence: 'officers are standing and on horses to form a line across a town center'. Find frequency of the letter 's'
Example Output:
6