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

Ex Input:
Sentence: 'many white birds flying above a river that flows in front of a forest'. Find frequency of the letter 'a'

Ex Output:
5


Ex Input:
Sentence: 'a little boy that has a toothbrush in his mouth'. Find frequency of the letter 'r'

Ex Output:
1


Ex Input:
Sentence: 'group of both sitting out of water in front of the castle'. Find frequency of the letter 'f'

Ex Output:

4