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

Example Input: Sentence: 'group of snowboarders on a snowy trail overlooking a mountain'. Find frequency of the letter 'r'
Example Output: 5

Example Input: Sentence: 'three men are in a kitchen cooking and cutting through a dish'. Find frequency of the letter 'a'
Example Output: 4

Example Input: Sentence: 'a plate of vegetables and an entree'. Find frequency of the letter 'g'
Example Output:
1