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 Java8 (OpenJDK 1.8.0) (judge verdict: Wrong Answer)

import java.util.*;
public class Main {

	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);

		int n = sc.nextInt();
		int d = sc.nextInt();
		int a = sc.nextInt();
		
		Dis[] arr = new Dis[n];
		for(int i=0;i<n;i++) {
			int x = sc.nextInt();
			int h = sc.nextInt();
			long l = x-d;
			long r = x+d;
			if(l<0)
				l = 0;
			arr[i] = new Dis(l, r, (int)Math.ceil((double)h/a));
		}
		Arrays.sort(arr, new Comparator<Dis>() {

			@Override
			public int compare(Dis o1, Dis o2) {
				if(o1.l>o2.l)
					return 1;
				else if(o1.l==o2.l) {
					if(o1.r>o2.r)
						return 1;
					else if(o1.r==o2.r)
						return 0;
					else
						return -1;
				}
				else
					return -1;
			}
			
		});
		
		long ans = 0;
		long l = arr[0].l, r = arr[0].r, t = arr[0].t;
		for(int i=1;i<n;i++) {
			if(arr[i].l<=r) {
				r = arr[i].r;
				t = Math.max(t, arr[i].t);
			}
			else {
				ans+=t;
				l = arr[i].l;
				r = arr[i].r;
				t = arr[i].t;
			}
		}
		if(t!=0)
			ans+=t;
		
		System.out.println(ans);
	}

}
class Dis{
	long l, r;
	int t;
	Dis(long l, long r, int t){
		this.l = l;
		this.r = r;
		this.t = t;
	}
}