Given the task definition and input, reply with output. In this task, you need to count the occurrences of the given word in the given sentence.

Sentence: 'two horses are grazing in an enclosed area with  short grass thats bordered one one side by a section of shaggy grass and at the far end by a line of tall trees under a blue sky'. Count the occurrences of the word 'of' in the given sentence.

2