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: Wrong Answer)

//by szh
#include<bits/stdc++.h>
using namespace std;

#define pb push_back
#define fi first
#define SZ(x) (int((x).size()))
#define se second
#define ll long long
#define pq priority_queue
#define MP make_pair
#define pii pair<int,int>
#define mod 998244353
#define debug(x) cerr<<#x<<"="<<x<<'\n'

const int maxn=2e5+10;

int n,d,a;
pii mon[maxn];
int pos[maxn];
ll tree[maxn];

int lowbit(int x) {
	return x&(-x);
}

ll query(int pos) {
	ll res=0ll;
	while (pos) {
		res+=tree[pos];
		pos-=lowbit(pos);
	}
	return res;
}

void update(int pos,ll val) {
	while (pos<maxn) {
		tree[pos]+=val;
		pos+=lowbit(pos);
	}
}

int main(){
	scanf("%d%d%d",&n,&d,&a);
	d*=2;
	for (int i=1;i<=n;i++) scanf("%d%d",&mon[i].fi,&mon[i].se);
	sort(mon+1,mon+n+1);
	for (int i=1;i<=n;i++) {
		pos[i]=mon[i].fi;
		update(i,mon[i].se-mon[i-1].se);
	}
	ll ans=0;
	for (int i=1;i<=n;i++) {
		ll cur=query(i);
		if (cur<=0ll) continue;
		int cnt=(cur-1)/a+1;
		ans+=(ll)cnt;
		int rt=upper_bound(pos+i,pos+1+n,pos[i]+d)-pos-1;
		if (rt>i) update(i+1,-a*cnt),update(rt+1,a*cnt);
	}
	printf("%lld",ans);
	return 0;
}