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

Example Input: Sentence: 'a teddy bear is sitting on a heart shaped patio chair outside'. Find frequency of the letter 'i'
Example Output: 6

Example Input: Sentence: 'a man paddle boarding by a boat dock in a city'. Find frequency of the letter 'o'
Example Output: 3

Example Input: Sentence: 'a group of people with surfboards enjoying a small river'. Find frequency of the letter 'r'
Example Output:
5