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

Example Input: Sentence: 'there is no image here to provide a caption for'. Find frequency of the letter 'o'
Example Output: 5

Example Input: Sentence: 'a man skating down a stair rail in front of a bunch of people'. Find frequency of the letter 'o'
Example Output: 5

Example Input: Sentence: 'a man dives forward to catch a frisbee'. Find frequency of the letter 'e'
Example Output:
3