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++14 (GCC 5.4.1) (judge verdict: Time Limit Exceeded)

#include<stdio.h>
#include<algorithm>
using namespace std;
struct mons{
	long long int x;
	long long int h;
}m[200005];
bool cmp(mons a,mons b){
	return a.x<b.x;
}
int main(){
	long long int n,d,a;
	scanf("%lld %lld %lld",&n,&d,&a);
	d=d*2;
	for(int i=0;i<n;i++){
		scanf("%lld %lld",&m[i].x,&m[i].h);
	}
	sort(m,m+n,cmp);
	long long int sum=0;
	for(int i=0;i<n;i++){
		if(m[i].h>0){
			long long int cishu=m[i].h/a;
			if(m[i].h%a!=0){
				cishu++;
			}
			long long int gongji=a*cishu;
			sum+=cishu;
			long long int k=i;
			while(1){
				if(k>=n)break;
				if(m[k].x<=m[i].x+d){
					m[k].h-=gongji;
				}
				else break;
				k++;
			}	
		}
	}
	printf("%lld",sum);
}