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

Example Input: Sentence: 'a gourmet looking hamburger with fries on a dining table with some wine'. Find frequency of the letter 't'
Example Output: 4

Example Input: Sentence: 'a group of men standing on the side of a boat'. Find frequency of the letter 'o'
Example Output: 5

Example Input: Sentence: 'statute of a horse and rider on top of a block wall'. Find frequency of the letter 'a'
Example Output:
5