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

Example Input: Sentence: 'older man getting ready to swing his bat at a ball on a stand in front of a group of viewers'. Find frequency of the letter 'n'
Example Output: 7

Example Input: Sentence: 'a man playing baseball on a television set'. Find frequency of the letter 'l'
Example Output: 4

Example Input: Sentence: 'a skier is wearing a red jacket and is skiing down a hill'. Find frequency of the letter 'k'
Example Output:
3