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

Example Input: Sentence: 'there is a zebra that is walking enar a car'. Find frequency of the letter 'r'
Example Output: 4

Example Input: Sentence: 'a young boy standing in front of a counter with dougnuts'. Find frequency of the letter 'o'
Example Output: 6

Example Input: Sentence: 'a woman with short hair consumes a banana'. Find frequency of the letter 'i'
Example Output:
2