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

Example Input: Sentence: 'a picture of two men on a motorcycle in front of a railroad track'. Find frequency of the letter 'n'
Example Output: 4

Example Input: Sentence: 'an airplane is being towed away from the jet bridge'. Find frequency of the letter 's'
Example Output: 1

Example Input: Sentence: 'a stuffed penguin sits on an office desk beside a computer monitor'. Find frequency of the letter 'o'
Example Output:
5