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

Example Input: Sentence: 'a man that is on his cell phone'. Find frequency of the letter 's'
Example Output: 2

Example Input: Sentence: 'there are two men sitting on a couch playing video games'. Find frequency of the letter 'v'
Example Output: 1

Example Input: Sentence: 'a man riding a skateboard on the side of a planter'. Find frequency of the letter 'i'
Example Output:
3