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: Accepted)

#include <bits/stdc++.h>
    using namespace std;
    #define INF 2000000000
    typedef long long int ll;
    typedef pair<ll,ll> ii;
    typedef pair<ii,ll> iii;
    typedef vector<ll> vi;
    typedef vector<ii> vii;
    #define getbit(n,i) (((n)&(1<<(i)))!=0)
    #define setbit0(n,i) ((n)&(~(1<<(i))))
    #define setbit1(n,i) ((n)|(1<<(i)))
    #define togglebit(n,i) ((n)^(1<<(i)))
    #define lastone(n) ((n)&(-(n)))
    #define read freopen("debug\\in.txt","r",stdin)
    #define write freopen("debug\\out.txt","w",stdout)
    #define fi first
    #define se second
    #define getI(a) scanf("%d",&a)
    #define getII(a,b) scanf("%d%d",&a,&b)
    #define PI (acos(-1))
    #define fastread ios_base::sync_with_stdio(false);cin.tie(NULL)
    #define mod (1000000007)
    #define asz 500005
    #define pb push_back
    #define ppf pop_front
    #define ppb pop_back
    #define pf push_front
    queue<pair<ll,ll> >q;
    vector<pair<ll,ll> >v;
    void solver(){
      ll n,i,k,l,m,d,a;
      cin>>n>>d>>a;
      for(i=1;i<=n;i++){
        cin>>k>>l;
        v.pb({k,l});
      }
      sort(v.begin(),v.end());
      ///q.push(v[0].fi,(v[0].se+a-1)/a);
      ll sum=0;
      ll res=0;
      for(i=0;i<n;i++){
        while(!q.empty()&&v[i].fi-q.front().fi>0){
          sum-=q.front().se;
          q.pop();
        }
        ll temp=(v[i].se+a-1)/a;
        if(temp>sum)
        {
          temp-=sum;
          q.push(make_pair(v[i].fi+2*d,temp));
          sum+=temp;
          res+=temp;
          }
      }
      cout<<res;
    }
    int main()
    {
        fastread;
        solver();
        return 0;
    }