Problem:
Score : 600 points

Problem Statement

Silver Fox is fighting with N monsters.

The monsters are standing in a row, and we can assume them to be standing on a number line. The i-th monster, standing at the coordinate X_i, has the health of H_i.

Silver Fox can use bombs to attack the monsters.
Using a bomb at the coordinate x decreases the healths of all monsters between the coordinates x-D and x+D (inclusive) by A.
There is no way other than bombs to decrease the monster's health.

Silver Fox wins when all the monsters' healths become 0 or below.

Find the minimum number of bombs needed to win.

Constraints

1 \leq N \leq 2 \times 10^5

0 \leq D \leq 10^9

1 \leq A \leq 10^9

0 \leq X_i \leq 10^9

1 \leq H_i \leq 10^9

X_i are distinct.

All values in input are integers.

Input

Input is given from Standard Input in the following format:

N D A
X_1 H_1
:
X_N H_N

Output

Print the minimum number of bombs needed to win.

Sample Input 1

3 3 2
1 2
5 4
9 2

Sample Output 1

2

First, let us use a bomb at the coordinate 4 to decrease the first and second monsters' health by 2.

Then, use a bomb at the coordinate 6 to decrease the second and third monsters' health by 2.

Now, all the monsters' healths are 0.
We cannot make all the monsters' health drop to 0 or below with just one bomb.

Sample Input 2

9 4 1
1 5
2 4
3 3
4 2
5 1
6 2
7 3
8 4
9 5

Sample Output 2

5

We should use five bombs at the coordinate 5.

Sample Input 3

3 0 1
300000000 1000000000
100000000 1000000000
200000000 1000000000

Sample Output 3

3000000000

Watch out for overflow.

Solution in Kotlin (1.0.0) (judge verdict: Time Limit Exceeded)

fun main(args: Array<String>) {
    val (N, D, A) = readLine()!!.split(" ").map(String::toLong)
    val XH = (1..N).map { readLine()!!.split(" ").map(String::toLong) }.map { MutablePair(it[0], it[1]) }.toSortedSet()
    var ans = 0L
    while (XH.isNotEmpty()) {
        val target = XH.first()
        val num = (target.h + A - 1) / A
        val damage = A * num
        val area = D * 2 + target.x
        val array = XH.toTypedArray()
        for (enemy in array) {
            if (area < enemy.x) break
            enemy.h -= damage
            if (enemy.h <= 0) XH.remove(enemy)
        }
        ans += num
    }
    println(ans)
}

class MutablePair(var x: Long, var h: Long):Comparable<MutablePair> {
    override fun compareTo(other: MutablePair): Int {
        return (x - other.x).toInt()
    }
}