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

Example Input: Sentence: 'an airplane is being towed away from the jet bridge'. Find frequency of the letter 's'
Example Output: 1

Example Input: Sentence: 'an image of a cautionary construction sign'. Find frequency of the letter 'a'
Example Output: 5

Example Input: Sentence: 'a group of dogs that are standing in a kitchen'. Find frequency of the letter 't'
Example Output:
4