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

Example Input: Sentence: 'three men are in a kitchen cooking and cutting through a dish'. Find frequency of the letter 'a'
Example Output: 4

Example Input: Sentence: 'a couple of men sitting next to each other under an umbrella'. Find frequency of the letter 'n'
Example Output: 5

Example Input: Sentence: 'some males a playing with a white frisbee'. Find frequency of the letter 'w'
Example Output:
2