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

Example Input: Sentence: 'a black bear sitting in the grass taking a rest'. Find frequency of the letter 'a'
Example Output: 6

Example Input: Sentence: 'the small black and white dog holds onto a frisbee'. Find frequency of the letter 'h'
Example Output: 3

Example Input: Sentence: 'a person leaning over an unmade bed in a cluttered room'. Find frequency of the letter 't'
Example Output:
2