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

Example Input: Sentence: 'the meal is served and ready to be eaten'. Find frequency of the letter 'e'
Example Output: 8

Example Input: Sentence: 'a man using a video game controller in a room'. Find frequency of the letter 'n'
Example Output: 4

Example Input: Sentence: 'two zebras standing around while a third zebra drinks from a pond'. Find frequency of the letter 's'
Example Output:
3