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

Example Input: Sentence: 'a desk covered in notepads, a laptop and other devices'. Find frequency of the letter 'c'
Example Output: 2

Example Input: Sentence: 'a red and white train passing a train station'. Find frequency of the letter 'a'
Example Output: 7

Example Input: Sentence: 'a black and white photo of a person surfing. the picture is from underneath the water'. Find frequency of the letter 'h'
Example Output:
5