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

Example Input: Sentence: 'a man skating down a stair rail in front of a bunch of people'. Find frequency of the letter 'o'
Example Output: 5

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

Example Input: Sentence: 'a park bench sits near the grass under a large tree'. Find frequency of the letter 'c'
Example Output:
1