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

Example Input: Sentence: 'an orange cat sitting on the ground next to a  purse'. Find frequency of the letter 's'
Example Output: 2

Example Input: Sentence: 'two boys are doing skateboard jumps in the street'. Find frequency of the letter 's'
Example Output: 4

Example Input: Sentence: 'there are two people playing a video game'. Find frequency of the letter 'l'
Example Output:
2