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

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

Example Input: Sentence: 'an commercial airline airplane flies above the clouds'. Find frequency of the letter 'n'
Example Output: 3

Example Input: Sentence: 'two horses are standing beside a baby horse'. Find frequency of the letter 'e'
Example Output:
5