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

Example Input: Sentence: 'an audience watches as a bird spreads its wings'. Find frequency of the letter 'd'
Example Output: 3

Example Input: Sentence: 'a softball player holding the bat on first base'. Find frequency of the letter 't'
Example Output: 4

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