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: 'two men in black wet suits beside a white surfboard'. Find frequency of the letter 'e'
Example Output: 5

Example Input: Sentence: 'two hot young women in bikinis riding surfboards'. Find frequency of the letter 'r'
Example Output:
3