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: 'three people are gathered around sitting on a park bench'. Find frequency of the letter 'g'
Example Output: 2

Example Input: Sentence: 'a man that is jumping a skateboard on a rail'. Find frequency of the letter 'l'
Example Output:
1