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

Example Input: Sentence: 'a baby calf has a tag on its right ear'. Find frequency of the letter 'n'
Example Output: 1

Example Input: Sentence: 'a man holding a frisbee high into the air'. Find frequency of the letter 'a'
Example Output: 4

Example Input: Sentence: 'in the background,  a shaded blue mountain and soft blue sky, fronted by a boat on water, leads to a foregrounds of water spry and a big wave, bearing a man in a rolled up top and trunks, hunched over and riding a surf board'. Find frequency of the letter 'r'
Example Output:
14