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 chair and umbrella on a sandy beach'. Find frequency of the letter 's'
Example Output: 1

Example Input: Sentence: 'two vehicles cross under several street lights at night'. Find frequency of the letter 'e'
Example Output:
7