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 grapefruit, toast, coffee and other items are on a tray'. Find frequency of the letter 't'
Example Output: 6

Example Input: Sentence: 'a young woman in a blue dress walks with a young man'. Find frequency of the letter 'n'
Example Output:
5