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

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

Example Input: Sentence: 'a man in costume talking on a cell phone'. Find frequency of the letter 'm'
Example Output: 2

Example Input: Sentence: 'a dump truck going down the road by some houses'. Find frequency of the letter 't'
Example Output:
2