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

Example Input: Sentence: 'a man sitting on a motorcycle near two other men'. Find frequency of the letter 'r'
Example Output: 3

Example Input: Sentence: 'a cat sprawls out and sleeps on its back on a lounge chair with an acorn theme'. Find frequency of the letter 'r'
Example Output: 3

Example Input: Sentence: 'chefs in uniform prepare food in a commercial kitchen'. Find frequency of the letter 'a'
Example Output:
3