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 standing in front of a statue near a metal pole'. Find frequency of the letter 'f'
Example Output: 2

Example Input: Sentence: 'a couple of cows are in a field'. Find frequency of the letter 'w'
Example Output:
1