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

Example Input: Sentence: 'people playing wii bowling in a modern library'. Find frequency of the letter 'o'
Example Output: 3

Example Input: Sentence: 'a dog on a couch near many pillows'. Find frequency of the letter 'p'
Example Output: 1

Example Input: Sentence: 'there are two zebras taht are walking'. Find frequency of the letter 't'
Example Output:
4