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

Example Input: Sentence: 'two metal benches on an empty train platform'. Find frequency of the letter 'n'
Example Output: 4

Example Input: Sentence: 'a person dressed all in black snowboarding down a slope'. Find frequency of the letter 'e'
Example Output: 4

Example Input: Sentence: 'a man that has a sink and a t.v. on the wall'. Find frequency of the letter 'a'
Example Output:
8