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

Example Input: Sentence: 'man in a sleeping bag on the floor using a laptop computer'. Find frequency of the letter 'i'
Example Output: 3

Example Input: Sentence: 'a tennis player after hitting a shot back to the other side of the court'. Find frequency of the letter 'n'
Example Output: 3

Example Input: Sentence: 'people plating cake in an office meeting room'. Find frequency of the letter 'g'
Example Output:
2