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

Example Input: Sentence: 'a table topped with a cupcake on a picture of a cupcake'. Find frequency of the letter 'o'
Example Output: 3

Example Input: Sentence: 'a couple of items that are on the wall'. Find frequency of the letter 'm'
Example Output: 1

Example Input: Sentence: 'two bears giving each other a big hug'. Find frequency of the letter 'e'
Example Output:
3