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 bird outside standing on a table looking up'. Find frequency of the letter 'b'
Example Output: 2

Example Input: Sentence: 'an open oven containing two thick crust pizzas'. Find frequency of the letter 's'
Example Output:
2