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

Example Input: Sentence: 'a herd of giraffe walking across a field'. Find frequency of the letter 'k'
Example Output: 1

Example Input: Sentence: 'a room with a big screen television and a couch'. Find frequency of the letter 'h'
Example Output: 2

Example Input: Sentence: 'a selection of glass vases and pitchers for sale'. Find frequency of the letter 'r'
Example Output:
2