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 bus with two floors belonging to london country'. Find frequency of the letter 't'
Example Output: 4

Example Input: Sentence: 'a woman pushing her bike through a stream'. Find frequency of the letter 'a'
Example Output:
4