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

Example Input: Sentence: 'a young boy standing in front of a counter with dougnuts'. Find frequency of the letter 'o'
Example Output: 6

Example Input: Sentence: 'the skateboarder is jumping over a huge statue of a shoe'. Find frequency of the letter 'i'
Example Output: 2

Example Input: Sentence: 'a no stopping and a no parking sign on a pole with a brooklyn street sign in the background'. Find frequency of the letter 'i'
Example Output:
6