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: 'statues of 2 zebras and 1 giraffe in a doorway'. Find frequency of the letter 'a'
Example Output: 6

Example Input: Sentence: 'a small bathroom with a sink, mirror and a bathtub'. Find frequency of the letter 'i'
Example Output:
3