A robot moves along the number line.

It starts at $0$, slides forward one unit (to $1$), slides backwards $2$ units (to $-1$), then forward $3$, back $4$, and so on.

It slides alternately forward and backwards, with each slide one unit longer than the previous one.

Where is the robot after $2011$ slides?


If you liked this problem, here is an NRICH task which challenges you to use similar mathematical ideas.