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

Ex Input:
Sentence: 'a blue train on a track next to another track'. Find frequency of the letter 'c'

Ex Output:
2

Ex Input:
Sentence: 'bathroom scene with a black cat going behind the door'. Find frequency of the letter 'e'

Ex Output:
4

Ex Input:
Sentence: 'a dog is sitting next to a hole'. Find frequency of the letter 'h'

Ex Output:
1