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

Example Input: Sentence: 'a half eaten doughnut in a persons hand'. Find frequency of the letter 'h'
Example Output: 3

Example Input: Sentence: 'a little boy sitting on a bed in a bedroom'. Find frequency of the letter 'e'
Example Output: 3

Example Input: Sentence: 'three people are standing and throwing a frisbee'. Find frequency of the letter 'a'
Example Output:
4