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

Example Input: Sentence: 'there is a stop sign that has a reflection in a puddle'. Find frequency of the letter 'e'
Example Output: 5

Example Input: Sentence: 'man in a radio studio wearing a suit in front of a microphone'. Find frequency of the letter 't'
Example Output: 3

Example Input: Sentence: 'there is one piece of pizza on the paper plate'. Find frequency of the letter 'r'
Example Output:
2