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

Example Input: Sentence: 'a train sits on a railroad track as a metal awning covers it'. Find frequency of the letter 'l'
Example Output: 2

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: 'several colorful images are arranged in a three by three grid'. Find frequency of the letter 'e'
Example Output:
9