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

Example Input: Sentence: 'a small boat travels on the water next to the shore'. Find frequency of the letter 'a'
Example Output: 5

Example Input: Sentence: 'salad and vegetables with me on top of plate'. Find frequency of the letter 'o'
Example Output: 3

Example Input: Sentence: 'a man hitting a baseball at a baseball game'. Find frequency of the letter 'a'
Example Output:
10