21243번 - Exactly One Point 스페셜 저지다국어

There are $n$ segments on a line. You should place some points onto this line so that:

• every point is contained in at least one segment,
• every segment contains exactly one point.

The first line contains an integer $n$ ($1 \le n \le 200000$) --- the number of segments.

Each of the next $n$ lines contains two integers $L_i$ and $R_i$ ($L_i < R_i$) --- the endpoints of the $i$-th segment.

For convenience, all endpoints of all segments are even numbers from $0$ to $4n - 2$.

If it's impossible to place points in a required way, output "-1".

Otherwise, in the first line output an integer $m$ --- the number of points that should be placed onto the line.

In the next line, output $m$ distinct integers from $0$ to $4n - 2$ --- the coordinates of the points.

You don't have to minimize the number of points. If there are several possible solutions, output any of them.