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 C# (Mono 4.6.2.0) (judge verdict: Wrong Answer)

using System;
using System.Collections.Generic;
class Program
{
	static void Main(string[] args)
	{
		string[] str = Console.ReadLine().Split();
		int N = int.Parse(str[0]);
		int D = int.Parse(str[1]);
		int A = int.Parse(str[2]);
		int[][]B = new int[N][];
		for(var i=0;i<N;i++){
			string[] str2 = Console.ReadLine().Split();
			B[i] = new int[2]{int.Parse(str2[0]),int.Parse(str2[1])};
		}
		Array.Sort(B,(x,y)=>x[0]-y[0]);
		long ans = 0;
		long t = B[0][0]+D;
		long m = B[0][1];
		for(var i=1;i<N;i++){
			if(B[i][0]-D>t+D){
				ans += (m-1)/A+1;
				t = B[i][0]+D;
				m = B[i][1];
			} else {
				m = Math.Max(B[i][1],m);
			}
			//Console.Error.WriteLine(ans+" "+t+" "+m);
		}
		ans += (m-1)/A+1;
		Console.WriteLine(ans);
	}
}