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

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: 'the miniature, elaborate exhibit depicts an ancient boating scene'. Find frequency of the letter 'n'
Example Output: 6

Example Input: Sentence: 'two baseball players standing on a field preparing to go to their assigned positions'. Find frequency of the letter 'a'
Example Output:
7