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: Runtime Error)

#include <bits/stdc++.h>
#define rep(i, n) for (int i = 0; i < n; i++)
#define inf 10000000000
using namespace std;
using ll = long long;
int main()
{
    int n,d,a;
    cin >> n >> d >> a;
    vector<int> x(n);
    vector<int> dp(1000000001);
    rep(i,n){
        int y, z;
        cin >> y >> z;
        x[i] = y;
        dp[y] = z;
    }
    sort(x.begin(), x.end());
    ll cnt = 0;
    int left=0, right=0;
    int dame = 0;
    int bomb = 0;
    rep(i, n)
    {
        left = x[i];
        if(left <= right){
        if((dp[left] -= dame) <= 0){
                continue;
            }
        }
        right = left + 2 * d;
        bomb = (dp[left] + a - 1) / a;
        dame = bomb * a;
        cnt += bomb;
    }
    cout << cnt << endl;
}