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

Example Input: Sentence: 'a person on a skateboard going down a ramp'. Find frequency of the letter 'n'
Example Output: 4

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: 'there are two hot dogs on the plate, one half eaten'. Find frequency of the letter 'n'
Example Output:
3