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

Example Input: Sentence: 'a social occasion of people are uncovered before the open day'. Find frequency of the letter 'c'
Example Output: 4

Example Input: Sentence: 'a man in his car eating a donut'. Find frequency of the letter 'm'
Example Output: 1

Example Input: Sentence: 'a woman with a bun on a plate on top of some food in a bucket'. Find frequency of the letter 'h'
Example Output:
1