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

Example Input: Sentence: 'silhouette of person playing frisbee with moutain and sunset'. Find frequency of the letter 'i'
Example Output: 5

Example Input: Sentence: 'a pizza is in a box on a table outside'. Find frequency of the letter 'u'
Example Output: 1

Example Input: Sentence: 'a baseball player holds a bat in his hand'. Find frequency of the letter 'h'
Example Output:
3