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

Example Input: Sentence: 'a blue train on a track next to another track'. Find frequency of the letter 'c'
Example Output: 2

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: 'people feast away over a food filled table'. Find frequency of the letter 't'
Example Output:
2