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

Example Input: Sentence: 'a young girl skiing down a snowy mountain'. Find frequency of the letter 'l'
Example Output: 1

Example Input: Sentence: 'a group of men standing on the side of a boat'. Find frequency of the letter 'o'
Example Output: 5

Example Input: Sentence: 'a man on a skateboard performs a trick for the camera'. Find frequency of the letter 'h'
Example Output:
1