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

Example Input: Sentence: 'two people in a snowy place near a wooden fence'. Find frequency of the letter 'n'
Example Output: 5

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 sitting on a couch after playing pool'. Find frequency of the letter 'l'
Example Output:
2