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

Example Input: Sentence: 'a train going over a bridge with some buildings in the background'. Find frequency of the letter 'd'
Example Output: 3

Example Input: Sentence: 'a tennis player after hitting a shot back to the other side of the court'. Find frequency of the letter 'n'
Example Output: 3

Example Input: Sentence: 'a cutting board with a slice of pizza on it'. Find frequency of the letter 'o'
Example Output:
3