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

Example Input: Sentence: 'two zebras standing around while a third zebra drinks from a pond'. Find frequency of the letter 's'
Example Output: 3

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 man holding a wii controller above his head and laughing'. Find frequency of the letter 'o'
Example Output:
4