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

Example Input: Sentence: 'a train going over a bridge with some buildings in the background'. Find frequency of the letter 'd'
Example Output: 3

Example Input: Sentence: 'a silver train crossing the road on tracks'. Find frequency of the letter 'r'
Example Output: 5

Example Input: Sentence: 'a hand holding a half eaten donut covered in coconut'. Find frequency of the letter 'e'
Example Output:
4