In this task, you need to count the occurrences of the given word in the given sentence.

Example Input: Sentence: 'a snowboarder in a gray jacket is doing a trick'. Count the occurrences of the word 'a' in the given sentence.
Example Output: 3

Example Input: Sentence: 'picture of a tab tub where the wall is open exposing wood studs and a shop vacuum is in the tub'. Count the occurrences of the word 'a' in the given sentence.
Example Output: 2

Example Input: Sentence: 'a train with grafitti sitting on the tracks'. Count the occurrences of the word 'a' in the given sentence.
Example Output:
1