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

Example Input: Sentence: 'a bathroom with a sink, counter top, and mirror is shown'. Find frequency of the letter 'i'
Example Output: 4

Example Input: Sentence: 'a bed with a dog laying on it next to two laptop computers'. Find frequency of the letter 'n'
Example Output: 3

Example Input: Sentence: 'a red bentley sits at a stop light by big ben in london'. Find frequency of the letter 'h'
Example Output:
1