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

Example Input: Sentence: 'a double-decker bus covered in children's illustrations driving next to a van'. Find frequency of the letter 'a'
Example Output: 4

Example Input: Sentence: 'a mixed bouquet of flowers in a glass vase set before a window'. Find frequency of the letter 'w'
Example Output: 3

Example Input: Sentence: 'half of a bus with stickers all over it'. Find frequency of the letter 'h'
Example Output:
2