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

Ex Input:
Sentence: 'two people under an umbrella while walking down a sidewalk'. Find frequency of the letter 'i'

Ex Output:
3

Ex Input:
Sentence: 'a man riding on the back of a white horse next to other horses'. Find frequency of the letter 'o'

Ex Output:
6

Ex Input:
Sentence: 'a bowl of food on plate on a table'. Find frequency of the letter 'l'

Ex Output:
3