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

Example Input: Sentence: 'a giraffe shares a habitat with a zebra, an ostrich, and an okapi'. Find frequency of the letter 'e'
Example Output: 3

Example Input: Sentence: 'a man skating down a stair rail in front of a bunch of people'. Find frequency of the letter 'o'
Example Output: 5

Example Input: Sentence: 'a man with his friend throwing a frisbee'. Find frequency of the letter 'o'
Example Output:
1