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

Example Input: Sentence: 'four women around a man sitting in front of a cake'. Find frequency of the letter 'o'
Example Output: 5

Example Input: Sentence: 'a train station with a train pulling into the platform'. Find frequency of the letter 'i'
Example Output: 6

Example Input: Sentence: 'a woman is laying on top of a suitcase'. Find frequency of the letter 'a'
Example Output:
5