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

Example Input: Sentence: 'a man with headphones on is hunched over a sink while being recorded'. Find frequency of the letter 'd'
Example Output: 4

Example Input: Sentence: 'an commercial airline airplane flies above the clouds'. Find frequency of the letter 'n'
Example Output: 3

Example Input: Sentence: 'there are three zebras standing next to each other'. Find frequency of the letter 't'
Example Output:
6