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

Example Input: Sentence: 'a red and white train passing a train station'. Find frequency of the letter 'a'
Example Output: 7

Example Input: Sentence: 'some giraffes moving across a big grass field'. Find frequency of the letter 'e'
Example Output: 3

Example Input: Sentence: 'young children are sitting in front of a large cake'. Find frequency of the letter 'n'
Example Output:
5