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

Example Input: Sentence: 'a view from a bathroom shows an open door'. Find frequency of the letter 'o'
Example Output: 7

Example Input: Sentence: 'a woman throwing a frisbee on a field'. Find frequency of the letter 's'
Example Output: 1

Example Input: Sentence: 'a group of people sitting on top of a bench'. Find frequency of the letter 'n'
Example Output:
3