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: 'one man grabbing another at a table while laughing'. Find frequency of the letter 'n'
Example Output: 5

Example Input: Sentence: 'the view of a room with a king sized bed'. Find frequency of the letter 'a'
Example Output:
2