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

Example Input: Sentence: 'a woman in a messy kitchen with a child'. Find frequency of the letter 'a'
Example Output: 4

Example Input: Sentence: 'a line of people on motorcycles on the road'. Find frequency of the letter 'e'
Example Output: 5

Example Input: Sentence: 'a woman cross country skiing in a competition'. Find frequency of the letter 's'
Example Output:
3