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

Example Input: Sentence: 'a teddy bear is sitting on a heart shaped patio chair outside'. Find frequency of the letter 'i'
Example Output: 6

Example Input: Sentence: 'a small boat and a large boat on a river near a city'. Find frequency of the letter 'a'
Example Output: 10

Example Input: Sentence: 'a black bear sitting down by a tree'. Find frequency of the letter 'o'
Example Output:
1