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

Ex Input:
Sentence: 'a chair and umbrella on a sandy beach'. Find frequency of the letter 's'

Ex Output:
1

Ex Input:
Sentence: 'a bedroom with a neatly made bed and a laptop'. Find frequency of the letter 'l'

Ex Output:
2

Ex Input:
Sentence: 'a tabby cat perched in the open window of old, rusty car'. Find frequency of the letter 'c'

Ex Output:
3