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

Example Input: Sentence: 'a street scene looking at the back of the stop sign'. Find frequency of the letter 't'
Example Output: 6

Example Input: Sentence: 'people waiting on a train platform in a wooded area'. Find frequency of the letter 'p'
Example Output: 3

Example Input: Sentence: 'a man that is laying down on a surfboard'. Find frequency of the letter 'i'
Example Output:
2