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

Example Input: Sentence: 'a man swinging a bat while playing baseball'. Find frequency of the letter 'm'
Example Output: 1

Example Input: Sentence: 'the skateboarder has on red and black sneakers and is riding a skateboard'. Find frequency of the letter 'g'
Example Output: 1

Example Input: Sentence: 'a white plate has mash potatoes, broccoli, and a meet dish on it'. Find frequency of the letter 'o'
Example Output:
5