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

Example Input: Sentence: 'a group of people stand inside of a big room'. Find frequency of the letter 'o'
Example Output: 6

Example Input: Sentence: 'young girl playing with refrigerator magnet while kneeling on floor'. Find frequency of the letter 'n'
Example Output: 6

Example Input: Sentence: 'a room with a couch, chair and a hospital bed'. Find frequency of the letter 'o'
Example Output:
4