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 (GCC 5.4.1) (judge verdict: Wrong Answer)


#include <stdio.h>
#include <stdlib.h>
#include <math.h>
long long MIN(long long a,long long b){
    return a<b?a:b;
}
typedef struct{
    long long int x;
    long long int h;
}monster;
int compare(const void *a,const void *b){
    monster p=*(monster *)a,q=*(monster *)b;
    long long* s=p.x,t=q.x;
    if(s>t)return 1;
    return -1;
}
int main()
{   
    long long a,n,d;
    scanf("%lld%lld%lld",&n,&d,&a);
    monster M[n];
    for(int i=0;i<n;i++){
        scanf("%lld%lld",&M[i].x,&M[i].h);
    }
    long long ans=0;
    qsort(M,n,sizeof(monster),compare);
    for(int i=0;i<n;i++){
        int t=(M[i].h+a-1)/a;
        ans+=t;
        int j=1;
        while(M[i+j].x<=M[i].x+2*d){
            M[i+j].h-=t*a;
            j++;
        }
    }
    printf("%lld",ans);
    return 0;
}