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: 'small airplane in the process of taking off'. Find frequency of the letter 'e'
Example Output: 3

Example Input: Sentence: 'a bowl of vegetables in a soup like substance next to a spoon'. Find frequency of the letter 'a'
Example Output:
5