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

Example Input: Sentence: 'a plastic tray that has different foods inside of it'. Find frequency of the letter 't'
Example Output: 6

Example Input: Sentence: 'a flock of sheep grazing in a park'. Find frequency of the letter 'f'
Example Output: 2

Example Input: Sentence: 'a red, white, cream and tan colored traffic sign on the corner'. Find frequency of the letter 'd'
Example Output:
3