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

Ex Input:
Sentence: 'a red and white train passing a train station'. Find frequency of the letter 'a'

Ex Output:
7


Ex Input:
Sentence: 'a surfer wipes out on a huge wave'. Find frequency of the letter 'a'

Ex Output:
3


Ex Input:
Sentence: 'a bed with a blanket under an open window'. Find frequency of the letter 'd'

Ex Output:

3