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

Ex Input:
Sentence: 'a picture of a toilet and toilet paper in a bathroom'. Find frequency of the letter 'o'

Ex Output:
5

Ex Input:
Sentence: 'a microwave oven with a yellow note on the door'. Find frequency of the letter 'w'

Ex Output:
3

Ex Input:
Sentence: 'a couple of young adults with a body board at the beach'. Find frequency of the letter 's'

Ex Output:
1