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

Example Input: Sentence: 'a bedroom with a neatly made bed and a laptop'. Find frequency of the letter 'l'
Example Output: 2

Example Input: Sentence: 'there is a baseball game on and a few people are watching'. Find frequency of the letter 'i'
Example Output: 2

Example Input: Sentence: 'group of mixed fruits sitting inside a metal basket'. Find frequency of the letter 'i'
Example Output:
6