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

Example Input: Sentence: 'a bird in a tree looking up at branches'. Find frequency of the letter 'e'
Example Output: 3

Example Input: Sentence: 'a man flying through the air on top of a skateboard'. Find frequency of the letter 'h'
Example Output: 3

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