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

Example Input: Sentence: 'a man sitting in front of a laptop computer next to a child'. Find frequency of the letter 'e'
Example Output: 2

Example Input: Sentence: 'there is no image here to provide a caption for'. Find frequency of the letter 'o'
Example Output: 5

Example Input: Sentence: 'a giant white teddy bears sits on an entryway'. Find frequency of the letter 'n'
Example Output:
4