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

Example Input: Sentence: 'a sheep in a field in front of other sheep'. Find frequency of the letter 'f'
Example Output: 3

Example Input: Sentence: 'a burning wood stove has a tea pot sitting on top of it and is full of wood'. Find frequency of the letter 'a'
Example Output: 5

Example Input: Sentence: 'a person on a skateboard by a wall'. Find frequency of the letter 't'
Example Output:
1