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

Example Input: Sentence: 'there is a sandwich with a lot of meat cut in half'. Find frequency of the letter 'a'
Example Output: 5

Example Input: Sentence: 'a man riding on the back of a white horse next to other horses'. Find frequency of the letter 'o'
Example Output: 6

Example Input: Sentence: 'there are several giraffes standing near some trees'. Find frequency of the letter 'a'
Example Output:
5