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

Example Input: Sentence: 'two people are riding on canoes down a river'. Find frequency of the letter 'e'
Example Output: 5

Example Input: Sentence: 'a number of small boats on a beach near an umbrella'. Find frequency of the letter 's'
Example Output: 2

Example Input: Sentence: 'a picture of two men on a motorcycle in front of a railroad track'. Find frequency of the letter 'n'
Example Output:
4