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

Ex Input:
Sentence: 'a woman walking with a child that is in a wheelchair'. Find frequency of the letter 'k'

Ex Output:
1

Ex Input:
Sentence: 'a police officer who is riding on a horse'. Find frequency of the letter 'p'

Ex Output:
1

Ex Input:
Sentence: 'a dog enjoys chewing on a carrot in the living room'. Find frequency of the letter 'o'

Ex Output:
6