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

Example Input: Sentence: 'young girl playing with refrigerator magnet while kneeling on floor'. Find frequency of the letter 'n'
Example Output: 6

Example Input: Sentence: 'a plane with smoke coming from the tips of both wings'. Find frequency of the letter 'c'
Example Output: 1

Example Input: Sentence: 'a woman talking on a cell phone next to a person wearing a yellow shirt'. Find frequency of the letter 'm'
Example Output:
1