Answer
This statement does not make sense.
Work Step by Step
There are no "negative" sheep. The position of the sheep is negative with respect to the ${{\left( n \right)}^{\text{th}}}$ sheep.
Note that here $ n-1$, $ n $, and $ n+1$ in the suffixes indicate the ${{\left( n-1 \right)}^{\text{th}}}$, ${{\left( n \right)}^{\text{th}}}$ and ${{\left( n+1 \right)}^{\text{th}}}$ sheep, and not the number of sheep.
Thus the statement does not make sense.
