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

Example Input: Sentence: 'an old boat on a beach near a large body of water'. Find frequency of the letter 'a'
Example Output: 8

Example Input: Sentence: 'a plastic plastic plate topped with three different type of food'. Find frequency of the letter 'l'
Example Output: 3

Example Input: Sentence: 'the kitchen has a built-in oven and microwave next to a tall built-in wine rack'. Find frequency of the letter 'e'
Example Output:
6