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: Accepted)

using Yorukatsu019.Questions;
using Yorukatsu019.Extensions;
using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Text;

namespace Yorukatsu019.Questions
{
    /// <summary>
    /// ABC153 F
    /// </summary>
    public class QuestionE : AtCoderQuestionBase
    {
        public override IEnumerable<object> Solve(TextReader inputStream)
        {
            var nda = inputStream.ReadIntArray();
            var monsterCount = nda[0];
            long bombRadius = nda[1];
            long bombAttack = nda[2];

            var monsters = new Monster[monsterCount];
            for (int i = 0; i < monsterCount; i++)
            {
                var xh = inputStream.ReadIntArray();
                monsters[i] = new Monster(xh[0], xh[1]);
            }
            Array.Sort(monsters);

            long totalBombCount = 0;
            long totalDamage = 0;
            var brasts = new Queue<Brast>();
            foreach (var monster in monsters)
            {
                while (brasts.Any() && brasts.Peek().EndPosition < monster.Position)
                {
                    var brast = brasts.Dequeue();
                    totalDamage -= brast.Damage;
                }

                var currentMonsterHp = monster.HP - totalDamage;
                if (currentMonsterHp > 0)
                {
                    var bombCount = (int)Math.Ceiling((double)currentMonsterHp / bombAttack);
                    totalDamage += bombAttack * bombCount;
                    totalBombCount += bombCount;

                    var brast = new Brast(bombAttack * bombCount, monster.Position + bombRadius * 2);
                    brasts.Enqueue(brast);
                }
            }

            yield return totalBombCount;
        }
    }

    struct Monster : IComparable<Monster>
    {
        public int Position { get; }
        public int HP { get; }

        public Monster(int position, int hp)
        {
            Position = position;
            HP = hp;
        }

        public int CompareTo(Monster other) => Position.CompareTo(other.Position);
    }

    struct Brast
    {
        public long Damage { get; }
        public long EndPosition { get; }

        public Brast(long damage, long endPosition)
        {
            Damage = damage;
            EndPosition = endPosition;
        }
    }
}


namespace Yorukatsu019
{
    class Program
    {
        static void Main(string[] args)
        {
            IAtCoderQuestion question = new QuestionE();    // 問題に合わせて書き換え
            var answers = question.Solve(Console.In);
            foreach (var answer in answers)
            {
                Console.WriteLine(answer);
            }
        }
    }
}

#region Base Classes

namespace Yorukatsu019.Questions
{

    public interface IAtCoderQuestion
    {
        IEnumerable<object> Solve(string input);
        IEnumerable<object> Solve(TextReader inputStream);
    }

    public abstract class AtCoderQuestionBase : IAtCoderQuestion
    {
        public IEnumerable<object> Solve(string input)
        {
            var stream = new MemoryStream(Encoding.Unicode.GetBytes(input));
            var reader = new StreamReader(stream, Encoding.Unicode);

            return Solve(reader);
        }

        public abstract IEnumerable<object> Solve(TextReader inputStream);
    }

}

#endregion

#region Extensions

namespace Yorukatsu019.Extensions
{
    internal static class TextReaderExtensions
    {
        internal static int ReadInt(this TextReader reader) => int.Parse(ReadString(reader));
        internal static long ReadLong(this TextReader reader) => long.Parse(ReadString(reader));
        internal static double ReadDouble(this TextReader reader) => double.Parse(ReadString(reader));
        internal static string ReadString(this TextReader reader) => reader.ReadLine();

        internal static int[] ReadIntArray(this TextReader reader, char separator = ' ') => ReadStringArray(reader, separator).Select(int.Parse).ToArray();
        internal static long[] ReadLongArray(this TextReader reader, char separator = ' ') => ReadStringArray(reader, separator).Select(long.Parse).ToArray();
        internal static double[] ReadDoubleArray(this TextReader reader, char separator = ' ') => ReadStringArray(reader, separator).Select(double.Parse).ToArray();
        internal static string[] ReadStringArray(this TextReader reader, char separator = ' ') => reader.ReadLine().Split(separator);
    }
}

#endregion