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

Example Input: Sentence: 'an elephant with broken tusks stands in front of a hill and boulders'. Find frequency of the letter 'n'
Example Output: 7

Example Input: Sentence: 'town clocks affixed to a tall, brick building'. Find frequency of the letter 'r'
Example Output: 1

Example Input: Sentence: 'a living room that has some furniture in it'. Find frequency of the letter 'a'
Example Output:
3