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

Example Input: Sentence: 'the bathroom is in need of remodel'. Find frequency of the letter 'e'
Example Output: 5

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

Example Input: Sentence: 'each of the passengers departing the metro train is carrying a luggage bag'. Find frequency of the letter 'r'
Example Output:
6