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

Example Input: Sentence: 'a group of tennis players on a tennis court'. Find frequency of the letter 'o'
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: 'two double decker buses parked by a bike rack'. Find frequency of the letter 't'
Example Output:
1