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

Ex Input:
Sentence: 'a person riding a camel in front of a truck'. Find frequency of the letter 't'

Ex Output:
2

Ex Input:
Sentence: 'a tennis player that is running to get the tennis ball'. Find frequency of the letter 't'

Ex Output:
7

Ex Input:
Sentence: 'three people walking inside a gate, one of them has skis and poles in her hand'. Find frequency of the letter 'n'

Ex Output:
6