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

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

Ex Output:
5

Ex Input:
Sentence: 'a young boy wearing black and holding a skateboard on the sidewalk'. Find frequency of the letter 'n'

Ex Output:
5

Ex Input:
Sentence: 'a boat goes across the water in front of buildings'. Find frequency of the letter 'a'

Ex Output:
4