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

Ex Input:
Sentence: 'a giraffe sticking it's tongue out near a window'. Find frequency of the letter 'a'

Ex Output:
4

Ex Input:
Sentence: 'a man riding a skateboard up the side of a ramp'. Find frequency of the letter 'u'

Ex Output:
1

Ex Input:
Sentence: 'there is a white dog sitting on the ground and a cow walking by'. Find frequency of the letter 'a'

Ex Output:
4