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

Example Input: Sentence: 'a full plate of food that includes rice and broccoli and carrots'. Find frequency of the letter 's'
Example Output: 2

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 man with a skateboard that is in the street'. Find frequency of the letter 'o'
Example Output:
1