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

Example Input: Sentence: 'a cat sprawls out and sleeps on its back on a lounge chair with an acorn theme'. Find frequency of the letter 'r'
Example Output: 3

Example Input: Sentence: 'a river with a long dragon boat in it and rowers on the boat in sets of two, along with two other people'. Find frequency of the letter 'h'
Example Output: 4

Example Input: Sentence: 'a man is holding a baseball bat swinging at the ball'. Find frequency of the letter 'i'
Example Output:
4