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

Example Input: Sentence: 'a flock of sheep grazing in a park'. Find frequency of the letter 'f'
Example Output: 2

Example Input: Sentence: 'an commercial airline airplane flies above the clouds'. Find frequency of the letter 'n'
Example Output: 3

Example Input: Sentence: 'underside of a passenger jet flying low to the ground at night'. Find frequency of the letter 'n'
Example Output:
5

