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

Example Input: Sentence: 'a man is water skiing on a river'. Find frequency of the letter 'r'
Example Output: 3

Example Input: Sentence: 'a woman and a child walking down a street with a kite'. Find frequency of the letter 'n'
Example Output: 4

Example Input: Sentence: 'a pair of men fighting over a frisbee in a grassy field'. Find frequency of the letter 'i'
Example Output:
6