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

Ex Input:
Sentence: 'a person is performing a stunt on a snowboard'. Find frequency of the letter 'p'

Ex Output:
2

Ex Input:
Sentence: 'a person on a skateboard going down a ramp'. Find frequency of the letter 'n'

Ex Output:
4

Ex Input:
Sentence: 'several people in river climbing on top of elephants'. Find frequency of the letter 'l'

Ex Output:
4