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 doorway looking into a small bathroom with white toilet'. Find frequency of the letter 'r'
Example Output: 2

Example Input: Sentence: 'a group of children at a table celebrating birthdays'. Find frequency of the letter 't'
Example Output:
4