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 person riding a surfboard in the middle of a wave'. Find frequency of the letter 'o'
Example Output: 3

Example Input: Sentence: 'a plate on a table with various foods'. Find frequency of the letter 'p'
Example Output:
1