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

Example Input: Sentence: 'small airplane in the process of taking off'. Find frequency of the letter 'e'
Example Output: 3

Example Input: Sentence: 'people seated on a table looking very happy'. Find frequency of the letter 'g'
Example Output: 1

Example Input: Sentence: 'a small microwave in a public eating area'. Find frequency of the letter 'r'
Example Output:
2