CF1601D.Difficult Mountain

A group of $n$ alpinists has just reached the foot of the mountain. The initial difficulty of climbing this mountain can be described as an integer $d$.

Each alpinist can be described by two integers $s$ and $a$, where $s$ is his skill of climbing mountains and $a$ is his neatness.

An alpinist of skill level $s$ is able to climb a mountain of difficulty $p$ only if $p \leq s$. As an alpinist climbs a mountain, they affect the path and thus may change mountain difficulty. Specifically, if an alpinist of neatness $a$ climbs a mountain of difficulty $p$ the difficulty of this mountain becomes $\max(p, a)$.

Alpinists will climb the mountain one by one. And before the start, they wonder, what is the maximum number of alpinists who will be able to climb the mountain if they choose the right order. As you are the only person in the group who does programming, you are to answer the question.

Note that after the order is chosen, each alpinist who can climb the mountain, must climb the mountain at that time.

一群 $n$ 名登山者刚刚抵达山脚。这座山初始的攀登难度可用一个整数 $d$ 来描述。

每名登山者可用两个整数 $s$ 和 $a$ 来刻画，其中 $s$ 表示其登山技能水平，$a$ 表示其整洁度（neatness）。

技能水平为 $s$ 的登山者仅当山的难度 $p$ 满足 $p \leq s$ 时，才能成功攀登一座难度为 $p$ 的山。当一名登山者攀登一座山时，其行为会影响路径，从而可能改变山的难度。具体而言，若一名整洁度为 $a$ 的登山者攀登了一座难度为 $p$ 的山，则该山的难度将变为 $\max(p, a)$。

登山者将依次逐个攀登此山。在开始攀登前，他们想知道：若能恰当地安排攀登顺序，最多有多少名登山者能够成功登顶？作为队伍中唯一会编程的人，你需要回答这个问题。

注意：一旦选定攀登顺序，所有在轮到其攀登时仍能满足当前山难度条件（即当前难度 $\leq$ 其技能 $s$）的登山者，都必须立即攀登该山。

The first line contains two integers $n$ and $d$ ($1 \leq n \leq 500\,000$; $0 \leq d \leq 10^9$) — the number of alpinists and the initial difficulty of the mountain.

Each of the next $n$ lines contains two integers $s_i$ and $a_i$ ($0 \leq s_i, a_i \leq 10^9$) that define the skill of climbing and the neatness of the $i$-th alpinist.

第一行包含两个整数 $n$ 和 $d$（$1 \leq n \leq 500\,000$；$0 \leq d \leq 10^9$）—— 分别表示登山者人数和山的初始难度。

接下来的 $n$ 行中，每行包含两个整数 $s_i$ 和 $a_i$（$0 \leq s_i, a_i \leq 10^9$），分别表示第 $i$ 位登山者的攀爬技能值和整洁度。

Print one integer equal to the maximum number of alpinists who can climb the mountain if they choose the right order to do so.

输出一个整数，表示如果登山者选择合适的攀登顺序，能够登上山顶的最大人数。

In the first example, alpinists $2$ and $3$ can climb the mountain if they go in this order. There is no other way to achieve the answer of $2$.

In the second example, alpinist $1$ is not able to climb because of the initial difficulty of the mountain, while alpinists $2$ and $3$ can go up in any order.

In the third example, the mountain can be climbed by alpinists $5$, $3$ and $4$ in this particular order. There is no other way to achieve optimal answer.

在第一个例子中，登山者 $2$ 和 $3$ 若按此顺序攀登，即可登上山顶。不存在其他方式达到答案 $2$。

在第二个例子中，登山者 $1$ 因山的初始难度而无法攀登，而登山者 $2$ 和 $3$ 可以以任意顺序登顶。

在第三个例子中，登山者 $5$、$3$ 和 $4$ 按此特定顺序可以登上山顶。不存在其他方式达到最优解。