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

Example Input: Sentence: 'the large modern train is on the train track'. Find frequency of the letter 'm'
Example Output: 1

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: 'the front of train makes its way along the tracks'. Find frequency of the letter 'k'
Example Output:
2