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

Example Input: Sentence: 'a little boy sitting on a bed in a bedroom'. Find frequency of the letter 'e'
Example Output: 3

Example Input: Sentence: 'a street scene looking at the back of the stop sign'. Find frequency of the letter 't'
Example Output: 6

Example Input: Sentence: 'man reaching for flying red disc in open area of park'. Find frequency of the letter 'n'
Example Output:
5