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

Example Input: Sentence: 'a bird is perched on a container in a person's hand'. Find frequency of the letter 'c'
Example Output: 2

Example Input: Sentence: 'silhouette of person playing frisbee with moutain and sunset'. Find frequency of the letter 'i'
Example Output: 5

Example Input: Sentence: 'the boy is entering the gate to begin his ski adventure'. Find frequency of the letter 'h'
Example Output:
3