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

Example Input: Sentence: 'three elephants walk up a dirt trail from the water'. Find frequency of the letter 'l'
Example Output: 3

Example Input: Sentence: 'town clocks affixed to a tall, brick building'. Find frequency of the letter 'r'
Example Output: 1

Example Input: Sentence: 'a couple of scooters are parked together in a building'. Find frequency of the letter 'i'
Example Output:
3