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

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

Example Input: Sentence: 'a group of people stand inside of a big room'. Find frequency of the letter 'o'
Example Output: 6

Example Input: Sentence: 'a man attempting to stand up on a surf board'. Find frequency of the letter 'a'
Example Output:
6