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 System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Reflection;
using System.Text;

namespace A
{
    class StartingPoint
    {
        static void Main(string[] args)
        {
            if (args?.Any() == true)
            {
                var assembly = Assembly.GetEntryAssembly();
                var resourceName = "AtCoderCSharp.In.txt";
                var stream = assembly.GetManifestResourceStream(resourceName);
                var streamReader = new StreamReader(stream, Encoding.UTF8);
                var str = streamReader.ReadToEnd();
                var textReader = new StringReader(str);
                Console.SetIn(textReader);
            }

            try
            {
                foreach (var s in new Program().Solve())
                {
                    Console.WriteLine(s);
                }
            }
            catch (Exception e)
            {
                Console.WriteLine(e);
                if (args?.Any() != true)
                    throw e;
            }

            if (args?.Any() == true)
                Console.ReadKey();
        }
    }

    class Program
    {
        public List<int> ScanInts(int count) => Enumerable.Range(0, count).Select(_ => ScanInt).ToList();
        public List<int> ScanInts(long count) => ScanInts((int)count);
        public List<long> ScanLongs(int count) => Enumerable.Range(0, count).Select(_ => Scan).ToList();
        public List<long> ScanLongs(long count) => ScanLongs((int)count);
        public int ScanInt => int.Parse(Str);
        public long Scan => long.Parse(Str);
        public double ScanDouble => (double)Scan;

        StringBuilder sb = new StringBuilder();

        public string Str => ScanStr();
        public string ScanStr()
        {
            sb.Clear();

            int r = 0;
            bool? b = null;
            do
            {
                r = Console.Read();
                b = IsBreak(r);
            } while (b == true);

            if (b == null)
                throw new Exception("input error: unexpected end of stream");

            do
            {
                sb.Append((char)r);
                r = Console.Read();
                b = IsBreak(r);
            } while (b == false);

            return sb.ToString();
        }

        bool? IsBreak(int c)
        {
            switch (c)
            {
                case -1:
                    return null;
                case (int)' ':
                case (int)'\n':
                case (int)'\r':
                    return true;
                default:
                    return false;
            }
        }

        public IEnumerable<int> Loop(int count) => Enumerable.Range(0, count);
        public IEnumerable<int> Loop(int from, int to) => Enumerable.Range(from, to - from + 1);

        public IEnumerable<long> Loop(long count) => Loop(0, count - 1);
        public IEnumerable<long> Loop(long from, long to)
        {
            for (long i = from; i <= to; i++)
                yield return i;
        }

        public string YesNo(bool isYes) => isYes ? "Yes" : "No";

        public HashSet<T> ToHashSet<T>(IEnumerable<T> ts) => new HashSet<T>(ts);

        public class Enemy
        {
            public long x;
            public long h;
        }

        int GetFirstIndexGreater(long x, ref List<long> listOrdered)
        {
            var count = listOrdered.Count;

            if (count == 0)
                return 0;

            if (listOrdered[count - 1] <= x)
                return count;

            int low = 0;
            int high = listOrdered.Count - 1;

            while (low < high)
            {
                var mid = (low + high) / 2;

                if (listOrdered[mid] > x)
                    high = mid;
                else
                    low = mid + 1;
            }

            return low;
        }

        int GetFirstIndexGreater(int x, ref List<int> listOrdered)
        {
            var count = listOrdered.Count;

            if (count == 0)
                return 0;

            if (listOrdered[count - 1] <= x)
                return count;

            int low = 0;
            int high = listOrdered.Count - 1;

            while (low < high)
            {
                var mid = (low + high) / 2;

                if (listOrdered[mid] > x)
                    high = mid;
                else
                    low = mid + 1;
            }

            return low;
        }

        int GetLastIndexLess(long x, ref List<long> listOrdered)
        {
            var count = listOrdered.Count;

            if (count == 0)
                return -1;

            if (listOrdered[0] >= x)
                return -1;

            int low = 0;
            int high = listOrdered.Count - 1;

            while (low < high)
            {
                var mid = (low + high + 1) / 2;

                if (listOrdered[mid] < x)
                    low = mid;
                else
                    high = mid - 1;
            }

            return low;
        }

        int GetLastIndexLess(int x, ref List<int> listOrdered)
        {
            var count = listOrdered.Count;

            if (count == 0)
                return -1;

            if (listOrdered[0] >= x)
                return -1;

            int low = 0;
            int high = listOrdered.Count - 1;

            while (low < high)
            {
                var mid = (low + high + 1) / 2;

                if (listOrdered[mid] < x)
                    low = mid;
                else
                    high = mid - 1;
            }

            return low;
        }

        public IEnumerable<string> Solve()
        {
            var n = ScanInt;
            var d = Scan;
            var a = Scan;
            var e = new List<Enemy>();


            foreach (var i in Loop(n))
            {
                e.Add(new Enemy() { x = Scan, h = Scan });
            }
            e = e.OrderBy(x => x.x).ToList();

            var xs = e.Select(x => x.x).ToList();

            var ds = new List<long>();
            var fs = new List<int>();
            var dd = 0L;

            foreach (var i in Loop(n))
            {
                var x = e[i].x;
                var xx = x + 2 * d;

                var li = GetLastIndexLess(xx + 1, ref xs);

                fs.AddRange(Enumerable.Range(0, li - fs.Count + 1).Select(_ => i));

                var bs = fs[i];
                var cd = dd - (bs > 0 ? ds[bs - 1] : 0);
                var ch = Math.Max(0, e[i].h - cd);
                var dn = ch / a * a + (ch % a > 0 ? a : 0);
                dd += dn;

                ds.Add(dd);
            }

            var res = dd / a;
            yield return res.ToString();
        }
    }
}