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

Example Input: Sentence: 'a microwave oven with a yellow note on the door'. Find frequency of the letter 'w'
Example Output: 3

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: 'outdoor furniture with tables and chairs under an open umbrella'. Find frequency of the letter 'a'
Example Output:
5