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

Ex Input:
Sentence: 'a man paddle boarding by a boat dock in a city'. Find frequency of the letter 'o'

Ex Output:
3

Ex Input:
Sentence: 'a train going over a bridge with some buildings in the background'. Find frequency of the letter 'd'

Ex Output:
3

Ex Input:
Sentence: 'two beds in a bedroom covered with blankets next to a window'. Find frequency of the letter 'd'

Ex Output:
4