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

Example Input: Sentence: 'a train on a train track going through a bunch of trees'. Find frequency of the letter 'u'
Example Output: 2

Example Input: Sentence: 'slices of orange are arranged on a plate'. Find frequency of the letter 's'
Example Output: 2

Example Input: Sentence: 'some colorful birds standing on some tree branches'. Find frequency of the letter 'i'
Example Output:
2