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

Example Input: Sentence: 'a man holding a brown paper bag kite'. Find frequency of the letter 'b'
Example Output: 2

Example Input: Sentence: 'a young boy wearing black and holding a skateboard on the sidewalk'. Find frequency of the letter 'n'
Example Output: 5

Example Input: Sentence: 'a bird perched on a thin tree limb'. Find frequency of the letter 'c'
Example Output:
1