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

Ex Input:
Sentence: 'a passenger train that is going over a bridge'. Find frequency of the letter 't'

Ex Output:
3

Ex Input:
Sentence: 'men are riding on motorcycles with a man riding in their carriers'. Find frequency of the letter 'r'

Ex Output:
8

Ex Input:
Sentence: 'three dogs of different colors laying on top of the sofa'. Find frequency of the letter 'r'

Ex Output:
3