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# (Mono 4.6.2.0) (judge verdict: Wrong Answer)

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace abc153_f
{
  class Program
  {
    static void Main(string[] args)
    {
      var N = CIn.ReadInt();
      var D = CIn.ReadInt();
      var A = CIn.ReadInt();
      var M = new Monster[N];

      for (int i = 0; i < N; i++)
      {
        M[i] = new Monster(CIn.ReadInt(), CIn.ReadInt());
      }

      Array.Sort(M, (a, b) => a.Pos - b.Pos);

      var ans = 0L;

      for (int i = 0; i < N; i++)
      {
        var t = M[i].Pos + D * 2;
        var l = 0;
        var r = N - 1;

        while (l != r)
        {
          var m = (r + l + 1) / 2;
          if (M[m].Pos <= t) l = m;
          else r = m - 1;
        }

        var c = (M[i].HP + A - 1) / A;
        ans += c;

        for (int j = i; j <= l; j++)
        {
          M[j].HP -= A * c;
        }
      }

      Console.WriteLine(ans);
    }
  }
  
  class Monster
  {
    public int Pos { get; }
    public int HP { get; set; }

    public Monster(int p, int h)
    {
      Pos = p;
      HP = h;
    }
  }

  static class CIn
  {
    private static readonly Queue<string> fieldBuffer = new Queue<string>();

    public static string Read()
    {
      if (fieldBuffer.Count == 0)
      {
        foreach (var field in Console.ReadLine().Split())
        {
          fieldBuffer.Enqueue(field);
        }
      }
      return fieldBuffer.Dequeue();
    }

    public static int ReadInt() => int.Parse(Read());
    public static long ReadLong() => long.Parse(Read());
    public static double ReadDouble() => double.Parse(Read());

    public static string[] ReadStringArray(long n)
    {
      var array = new string[n];
      for (var i = 0L; i < n; i++)
      {
        array[i] = Read();
      }
      return array;
    }

    public static int[] ReadIntArray(long n)
    {
      var array = new int[n];
      for (var i = 0L; i < n; i++)
      {
        array[i] = ReadInt();
      }
      return array;
    }

    public static long[] ReadLongArray(long n)
    {
      var array = new long[n];
      for (var i = 0L; i < n; i++)
      {
        array[i] = ReadLong();
      }
      return array;
    }

    public static double[] ReadDoubleArray(long n)
    {
      var array = new double[n];
      for (var i = 0L; i < n; i++)
      {
        array[i] = ReadDouble();
      }
      return array;
    }

    public static char[,] ReadCharMatrix(long h, long w)
    {
      var array = new char[h, w];
      for (var i = 0L; i < h; i++)
      {
        var l = Console.ReadLine().ToCharArray();
        for (var j = 0L; j < w; j++)
        {
          array[i, j] = l[j];
        }
      }
      return array;
    }
  }
}