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

Example Input: Sentence: 'a boy that is sitting at a table with pizza'. Find frequency of the letter 's'
Example Output: 2

Example Input: Sentence: 'some giraffes moving across a big grass field'. Find frequency of the letter 'e'
Example Output: 3

Example Input: Sentence: 'a glass a rope and a pair of scissors'. Find frequency of the letter 's'
Example Output:
6