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

Ex Input:
Sentence: 'two men sitting quietly on a park bench'. Find frequency of the letter 'a'

Ex Output:
2

Ex Input:
Sentence: 'two men in black wet suits beside a white surfboard'. Find frequency of the letter 'e'

Ex Output:
5

Ex Input:
Sentence: 'a pizza with cheese, olives, onions, and sauce'. Find frequency of the letter 'a'

Ex Output:
4