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

Example Input: Sentence: 'a small boat and a large boat on a river near a city'. Find frequency of the letter 'a'
Example Output: 10

Example Input: Sentence: 'two zebras feed on a plant through some wire mesh'. Find frequency of the letter 'o'
Example Output: 4

Example Input: Sentence: 'party cakes in plastic containers, with a cake with a plastic santa claus and reindeer on top'. Find frequency of the letter 's'
Example Output:
6