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

Example Input: Sentence: 'the large modern train is on the train track'. Find frequency of the letter 'm'
Example Output: 1

Example Input: Sentence: 'three men in military suits are sitting on a bench,'. Find frequency of the letter 'a'
Example Output: 3

Example Input: Sentence: 'three people are sitting on a bench looking over a pier'. Find frequency of the letter 'b'
Example Output:
1