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

Example Input: Sentence: 'a woman throwing a frisbee on a field'. Find frequency of the letter 's'
Example Output: 1

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 snowboarder riding on a half-pipe preforming a board grab'. Find frequency of the letter 'd'
Example Output:
3