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

Example Input: Sentence: 'a group of people having dinner and drinking wine'. Find frequency of the letter 'r'
Example Output: 3

Example Input: Sentence: 'statues of 2 zebras and 1 giraffe in a doorway'. Find frequency of the letter 'a'
Example Output: 6

Example Input: Sentence: 'an oriental man shows off his two cell phones'. Find frequency of the letter 'a'
Example Output:
3