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

Example Input: Sentence: 'a man pushing a cart full of bananas'. Find frequency of the letter 'u'
Example Output: 2

Example Input: Sentence: 'two men with colorful ties are posing for a picture'. Find frequency of the letter 'r'
Example Output: 4

Example Input: Sentence: 'a young boy wearing black and holding a skateboard on the sidewalk'. Find frequency of the letter 'n'
Example Output:
5