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: 'flower in a vase full of water sitting on a stand '. Find frequency of the letter 'l'
Example Output: 3

Example Input: Sentence: 'a male in a white shirt is riding a blue skateboard'. Find frequency of the letter 'e'
Example Output:
4