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

Example Input: Sentence: 'cars parked outside if a retail store'. Find frequency of the letter 'd'
Example Output: 2

Example Input: Sentence: 'a toilet filled with a brown liner and dirty water'. Find frequency of the letter 't'
Example Output: 5

Example Input: Sentence: 'a baseball player standing next to home plate'. Find frequency of the letter 'o'
Example Output:
2