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

Example Input: Sentence: 'a girl playing with a frisbee while people watch'. Find frequency of the letter 'e'
Example Output: 5

Example Input: Sentence: 'an audience watches as a bird spreads its wings'. Find frequency of the letter 'd'
Example Output: 3

Example Input: Sentence: 'a desk in a room with many books'. Find frequency of the letter 'k'
Example Output:
2