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

Example Input: Sentence: 'three men are in a kitchen cooking and cutting through a dish'. Find frequency of the letter 'a'
Example Output: 4

Example Input: Sentence: 'a man with a skateboard that is in the street'. Find frequency of the letter 'o'
Example Output: 1

Example Input: Sentence: 'two men playing a video game as other look on'. Find frequency of the letter 'h'
Example Output:
1