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

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 is taking a picture of a man and woman'. Find frequency of the letter 'a'
Example Output: 8

Example Input: Sentence: 'an oven with a cooked turkey sitting in it'. Find frequency of the letter 'o'
Example Output:
3