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HIIT Open 2018

Start:2018-05-26 11:00:00
End:2018-05-26 16:00:00

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CSES - HIIT Open 2018 - Knobs


Time limit:2.00 s
Memory limit:512 MB

You have a graph with $n$ nodes and $m$ edges. Each node has a knob that you can turn clockwise or counterclockwise. Each knob can be in one of three positions: 0°, 120°, 240°. Initially, knob $i$ is in position $a_i$.

For each knob $i$ you also know its final position, $b_i$. You will need to find a way to turn knobs so that: Note that you can turn the same knob many times, to any of the positions.


The first input line has two integers $n$ and $m$: the number of knobs and edges.

Then there are $n$ lines, each with two numbers $a_i$ and $b_i$. These indicate the current position $a_i$ and the final position $b_i$ of knob $i$.

Finally, there are $m$ lines, each with two numbers $i$ and $j$. This indicates that there is an edge between nodes $i$ and $j$; here $1 \le i < j \le n$.

The input configuration and the final configuration are valid: if there is an edge between knobs $i$ and $j$, then $a_i \ne a_j$ and $b_i \ne b_j$.

All inputs are such that if a solution exists, there is at least one solution with at most $10^5$ steps.


If there is a way to turn knobs so that you can reach the final configuration, print a sequence that shows how to do it. First print one line with a number $s$ (at most $5 \cdot 10^5$): the number of steps. Then print $s$ lines, each with a pair of numbers, $x_k$ and $p_k$: in step $k$ you turn knob $x_k$ to position $p_k$. Here $1 \le x_k \le n$ and $p_k$ is 0, 120, or 240.

If there is no way to solve the problem, print only one line with value $-1$.


4 4
0 0
120 240
240 120
120 240
1 2
2 3
3 4
1 4

3 0
4 240
2 240
3 120

The following illustration shows the solution with four steps. Here 0° points up, 120° down right and 240° down left.