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

Example Input: Sentence: 'a large motorcycle that has a man dressed in old clothing next to it'. Find frequency of the letter 'a'
Example Output: 6

Example Input: Sentence: 'a group of giraffes walk together by a hill'. Find frequency of the letter 'b'
Example Output: 1

Example Input: Sentence: 'a guy preforming a skateboard trick at a skate park'. Find frequency of the letter 'm'
Example Output:
1