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

Example Input: Sentence: 'a silver train crossing the road on tracks'. Find frequency of the letter 'r'
Example Output: 5

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 living room, with a laptop on the couch'. Find frequency of the letter 'o'
Example Output:
5