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

Example Input: Sentence: 'a man in a sweatshirt is taking his own picture'. Find frequency of the letter 'u'
Example Output: 1

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 plate topped with two pastries and a fork'. Find frequency of the letter 'p'
Example Output:
4