Score : $700$ points
You are given an integer $N$. Build an undirected graph with $N$ vertices with indices $1$ to $N$ that satisfies the following two conditions:
It can be proved that at least one such graph exists under the constraints of this problem.
Input is given from Standard Input in the following format:
In the first line, print the number of edges, $M$, in the graph you made. In the $i$-th of the following $M$ lines, print two integers $a_i$ and $b_i$, representing the endpoints of the $i$-th edge.
The output will be judged correct if the graph satisfies the conditions.
2 1 3 2 3