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

Example Input: Sentence: 'a large cooked pizza on a dining table'. Find frequency of the letter 'n'
Example Output: 3

Example Input: Sentence: 'a teddy bear is sitting on a heart shaped patio chair outside'. Find frequency of the letter 'i'
Example Output: 6

Example Input: Sentence: 'a man that is jumping a skateboard on a rail'. Find frequency of the letter 'l'
Example Output:
1