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: 'a woman walking with a child that is in a wheelchair'. Find frequency of the letter 'k'
Example Output: 1

Example Input: Sentence: 'small child sitting in front of a table with handmade pizzas'. Find frequency of the letter 'n'
Example Output:
4