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

Example Input: Sentence: 'a person on a sidewalk jumping with an umbrella'. Find frequency of the letter 'a'
Example Output: 5

Example Input: Sentence: 'some sheep are standing in a grassy field'. Find frequency of the letter 'n'
Example Output: 3

Example Input: Sentence: 'a man throwing red frisbees for his dog to catch mid air'. Find frequency of the letter 'h'
Example Output:
3