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 double decker transit bus parks by a blue on the side of the street in front of a brick building in the city'. Find frequency of the letter 't'
Example Output: 9

Example Input: Sentence: 'the man is jumping for joy by the fire hydrant'. Find frequency of the letter 'n'
Example Output:
3