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

Ex Input:
Sentence: 'an orange cat sitting on the ground next to a purse'. Find frequency of the letter 's'

Ex Output:
2

Ex Input:
Sentence: 'a gray airplane with metal petals on the wings takes off from an airport'. Find frequency of the letter 'i'

Ex Output:
4

Ex Input:
Sentence: 'a wood table topped with a candle and computer'. Find frequency of the letter 'a'

Ex Output:
5