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

Example Input: Sentence: 'two people in a snowy place near a wooden fence'. Find frequency of the letter 'n'
Example Output: 5

Example Input: Sentence: 'a large group of motorcycles parked along a street'. Find frequency of the letter 'a'
Example Output: 5

Example Input: Sentence: 'two men with colorful ties are posing for a picture'. Find frequency of the letter 'r'
Example Output:
4