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: '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 kitchen with two ranges and an industrial coffee maker'. Find frequency of the letter 'd'
Example Output:
2