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: 'a white bathroom sink sitting next to a toilet'. Find frequency of the letter 'i'
Example Output: 5

Example Input: Sentence: 'a female soldier showing something to small children'. Find frequency of the letter 'e'
Example Output:
5