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

Example Input: Sentence: 'a close up of a train on a track in the dark'. Find frequency of the letter 'a'
Example Output: 6

Example Input: Sentence: 'a man with glasses adjusting his tie in a mirror'. Find frequency of the letter 's'
Example Output: 5

Example Input: Sentence: 'a train that is going by some trees'. Find frequency of the letter 'o'
Example Output:
2