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

Example Input: Sentence: 'a man paddle boarding by a boat dock in a city'. Find frequency of the letter 'o'
Example Output: 3

Example Input: Sentence: 'man in a radio studio wearing a suit in front of a microphone'. Find frequency of the letter 't'
Example Output: 3

Example Input: Sentence: 'man sits behind a microphone at a news conference'. Find frequency of the letter 'h'
Example Output:
2