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

Example Input: Sentence: 'a little boy that has a toothbrush in his mouth'. Find frequency of the letter 'r'
Example Output: 1

Example Input: Sentence: 'two people are riding on canoes down a river'. Find frequency of the letter 'e'
Example Output: 5

Example Input: Sentence: 'a small cute cat sitting in the bathroom sink'. Find frequency of the letter 'u'
Example Output:
1