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

Example Input: Sentence: 'a group of people walking down a rain soaked street with umbrellas'. Find frequency of the letter 'w'
Example Output: 3

Example Input: Sentence: 'a plastic tray that has different foods inside of it'. Find frequency of the letter 't'
Example Output: 6

Example Input: Sentence: 'a person in a snow jacket and pants in skis'. Find frequency of the letter 's'
Example Output:
5