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

Example Input: Sentence: 'a red and white train passing a train station'. Find frequency of the letter 'a'
Example Output: 7

Example Input: Sentence: 'a man holding a frisbee high into the air'. Find frequency of the letter 'a'
Example Output: 4

Example Input: Sentence: 'a red cellphone a small cow and boat'. Find frequency of the letter 'd'
Example Output:
2