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

Example Input: Sentence: 'a picture of a toilet and toilet paper in a bathroom'. Find frequency of the letter 'o'
Example Output: 5

Example Input: Sentence: 'an airplane is being towed away from the jet bridge'. Find frequency of the letter 's'
Example Output: 1

Example Input: Sentence: 'kites waver in the air and include a man shaped flying object while people stand on the ground'. Find frequency of the letter 'w'
Example Output:
2