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

Ex Input:
Sentence: 'there is a baseball game on and a few people are watching'. Find frequency of the letter 'i'

Ex Output:
2

Ex Input:
Sentence: 'a small boy in a high chair eating a cupcake'. Find frequency of the letter 'i'

Ex Output:
4

Ex Input:
Sentence: 'there is one slice of pizza on the tray and a woman sitting behind a sign'. Find frequency of the letter 'd'

Ex Output:
2