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

Sentence: 'many white birds flying above a river that flows in front of a forest'. Find frequency of the letter 'a'
5

Sentence: 'a woman walking on a shore next to a river'. Find frequency of the letter 'v'
1

Sentence: 'two elephants walking on cement next to wall'. Find frequency of the letter 'n'

5