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

Example Input: Sentence: 'old double decker bus parked in a lot'. Find frequency of the letter 't'
Example Output: 1

Example Input: Sentence: 'a woman at a table checking her cell phone'. Find frequency of the letter 'a'
Example Output: 5

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