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

Example Input: Sentence: 'a laptop computer with pictures of giraffes on the homescreen'. Find frequency of the letter 'o'
Example Output: 5

Example Input: Sentence: 'a train on a train track going through a bunch of trees'. Find frequency of the letter 'u'
Example Output: 2

Example Input: Sentence: 'a yellow suv taxi cab driving past very tall buildings'. Find frequency of the letter 'y'
Example Output:
2