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

Example Input: Sentence: 'a group of people standing around a table filled with assorted electronic devices'. Find frequency of the letter 'w'
Example Output: 1

Example Input: Sentence: 'a plate of food, with broccoli and nuts'. Find frequency of the letter 'b'
Example Output: 1

Example Input: Sentence: 'people standing around in front of a bike store'. Find frequency of the letter 'a'
Example Output:
3