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 family out snow skiing on a blustery day'. Find frequency of the letter 'i'
Example Output: 3

Example Input: Sentence: 'a pink, white and blue bus is by a small building'. Find frequency of the letter 'd'
Example Output:
2