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

Example Input: Sentence: 'a railroad train pulling up to the train platform'. Find frequency of the letter 'o'
Example Output: 3

Example Input: Sentence: 'a tall building with antenna stands before a jet flying in the background'. Find frequency of the letter 't'
Example Output: 6

Example Input: Sentence: 'a street pole with signs and lights on it'. Find frequency of the letter 'i'
Example Output:
4