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

Example Input: Sentence: 'a girl that has a kite in the air'. Find frequency of the letter 'r'
Example Output: 2

Example Input: Sentence: 'a ready to eat pizza on a rack'. Find frequency of the letter 'r'
Example Output: 2

Example Input: Sentence: 'a yellow train coming down the train track'. Find frequency of the letter 'w'
Example Output:
2