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: 'a woman cross country skiing in a competition'. Find frequency of the letter 's'
Example Output: 3

Example Input: Sentence: 'a free-standing bathroom near an open door and table'. Find frequency of the letter 's'
Example Output:
1