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

Ex Input:
Sentence: 'a girl playing with a frisbee while people watch'. Find frequency of the letter 'e'

Ex Output:
5

Ex Input:
Sentence: 'person wearing all white leaned up against a wall with a yellow sign'. Find frequency of the letter 'a'

Ex Output:
8

Ex Input:
Sentence: 'a man eating a pastry with a woman leaning on him'. Find frequency of the letter 'a'

Ex Output:
8