How many sheep do you reckon there are?

A farmer was showing a schoolboy round his farm when they came to a field where the farmer’s sheep were grazing.
“How many sheep do you reckon there are?” asked the farmer proudly.
“Seven hundred and sixty-four,” replied the boy after four seconds.
The farmer gaped.
“That’s exactly right, boy. How did you count them so quickly?”
“Simple,” Said the boy genius. “I just counted the legs and divided by four.”