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

Example Input: Sentence: 'a baseball themed birthday cake is on a serving tray'. Find frequency of the letter 'g'
Example Output: 1

Example Input: Sentence: 'a woman and a child walking down a street with a kite'. Find frequency of the letter 'n'
Example Output: 4

Example Input: Sentence: 'a motor scooter with two adults and two children riding on it in a street'. Find frequency of the letter 't'
Example Output:
9