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

Example Input: Sentence: 'the interior of a living and dining room'. Find frequency of the letter 'i'
Example Output: 6

Example Input: Sentence: 'people waiting on a train platform in a wooded area'. Find frequency of the letter 'p'
Example Output: 3

Example Input: Sentence: 'a row of cars at street intersection under a traffic light'. Find frequency of the letter 's'
Example Output:
3