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

Example Input: Sentence: 'surfboard being displayed in stor window with clothes behind it'. Find frequency of the letter 't'
Example Output: 4

Example Input: Sentence: 'party cakes in plastic containers, with a cake with a plastic santa claus and reindeer on top'. Find frequency of the letter 's'
Example Output: 6

Example Input: Sentence: 'there is a bowl of granola and a banana on a plate'. Find frequency of the letter 'o'
Example Output:
4