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: 'a passenger train that is going over a bridge'. Find frequency of the letter 't'
Example Output: 3

Example Input: Sentence: 'this is a vintage german omnibus parked  in a vehicle show'. Find frequency of the letter 'g'
Example Output:
2