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

Ex Input:
Sentence: 'a person working at an airport, outside of an airplane'. Find frequency of the letter 'r'

Ex Output:
5

Ex Input:
Sentence: 'the little girl is flying her kite in the field'. Find frequency of the letter 'i'

Ex Output:
7

Ex Input:
Sentence: 'a couple of people tending to a conveyor belt with lots of donuts on it'. Find frequency of the letter 'r'

Ex Output:
1