CF1015A.Points in Segments

You are given a set of $n$ segments on the axis $Ox$, each segment has integer endpoints between $1$ and $m$ inclusive. Segments may intersect, overlap or even coincide with each other. Each segment is characterized by two integers $l_i$ and $r_i$ ($1 \le l_i \le r_i \le m$) — coordinates of the left and of the right endpoints.

Consider all integer points between $1$ and $m$ inclusive. Your task is to print all such points that don't belong to any segment. The point $x$ belongs to the segment $[l; r]$ if and only if $l \le x \le r$.

给你 $n$ 条位于 $Ox$ 轴上的线段，每条线段的端点均为 $1$ 到 $m$（含）之间的整数。这些线段可能相交、重叠，甚至完全重合。每条线段由两个整数 $l_i$ 和 $r_i$（满足 $1 \le l_i \le r_i \le m$）刻画，分别表示其左端点和右端点的坐标。

考虑 $1$ 到 $m$（含）之间的所有整数点。你的任务是输出所有不属于任何线段的点。点 $x$ 属于线段 $[l; r]$ 当且仅当 $l \le x \le r$。

The first line of the input contains two integers $n$ and $m$ ($1 \le n, m \le 100$) — the number of segments and the upper bound for coordinates.

The next $n$ lines contain two integers each $l_i$ and $r_i$ ($1 \le l_i \le r_i \le m$) — the endpoints of the $i$-th segment. Segments may intersect, overlap or even coincide with each other. Note, it is possible that $l_i=r_i$, i.e. a segment can degenerate to a point.

输入的第一行包含两个整数 $n$ 和 $m$（$1 \le n, m \le 100$）——分别表示线段的数量以及坐标的上界。

接下来的 $n$ 行，每行包含两个整数 $l_i$ 和 $r_i$（$1 \le l_i \le r_i \le m$）——表示第 $i$ 条线段的两个端点。这些线段可能相交、重叠，甚至完全重合。注意，可能出现 $l_i = r_i$ 的情况，即一条线段可能退化为一个点。

In the first line print one integer $k$ — the number of points that don't belong to any segment.

In the second line print exactly $k$ integers in any order — the points that don't belong to any segment. All points you print should be distinct.

If there are no such points at all, print a single integer $0$ in the first line and either leave the second line empty or do not print it at all.

第一行输出一个整数 $k$ —— 不属于任何线段的点的个数。

第二行按任意顺序恰好输出 $k$ 个整数 —— 这些不属于任何线段的点。你输出的所有点必须互不相同。

如果不存在这样的点，则在第一行输出单个整数 $0$，第二行可留空或完全不输出。

In the first example the point $1$ belongs to the second segment, the point $2$ belongs to the first and the second segments and the point $5$ belongs to the third segment. The points $3$ and $4$ do not belong to any segment.

In the second example all the points from $1$ to $7$ belong to the first segment.

在第一个例子中，点 $1$ 属于第二条线段，点 $2$ 属于第一条和第二条线段，点 $5$ 属于第三条线段。点 $3$ 和 $4$ 不属于任何线段。

在第二个例子中，从 $1$ 到 $7$ 的所有点都属于第一条线段。