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

Example Input: Sentence: 'a snowboarder making a jump of a snowy ledge'. Find frequency of the letter 'o'
Example Output: 4

Example Input: Sentence: 'a bunch of broken down trains sit in a yard'. Find frequency of the letter 'n'
Example Output: 5

Example Input: Sentence: 'a table has a television and some plants on top of it'. Find frequency of the letter 'n'
Example Output:
4