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

Example Input: Sentence: 'a airplane view of a big factory some people and cars'. Find frequency of the letter 'f'
Example Output: 2

Example Input: Sentence: 'a few giraffes standing in a field looking around'. Find frequency of the letter 'e'
Example Output: 3

Example Input: Sentence: 'several animals with large horns eat while a zebra drinks from a pond near a tree'. Find frequency of the letter 'a'
Example Output:
10