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

Example Input: Sentence: 'a bathroom with a sink, counter top, and mirror is shown'. Find frequency of the letter 'i'
Example Output: 4

Example Input: Sentence: 'a few giraffes standing in a field looking around'. Find frequency of the letter 'e'
Example Output: 3

Example Input: Sentence: 'a train pulls up as a man stands waiting alongside snack machines'. Find frequency of the letter 'g'
Example Output:
2