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

Example Input: Sentence: 'hand holding a white calculator inside a store'. Find frequency of the letter 'o'
Example Output: 3

Example Input: Sentence: 'a batter, catcher and umpire waiting for the pitch and a major league baseball game'. Find frequency of the letter 'c'
Example Output: 3

Example Input: Sentence: 'friends and family at a happy luncheon in a restaurant'. Find frequency of the letter 'r'
Example Output:
3