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;
using System.Collections.Generic;
using System.Diagnostics;
using System.IO;
using System.Linq;
using System.Numerics;
using System.Runtime.InteropServices;
using System.Text;
using System.Text.RegularExpressions;
using System.Threading.Tasks;
using static System.Math;
using static Math2;
using static io;
using static Utils;

public class CP {
  public void Solve () {
    var o = Longs;
    var N = (int) o[0];
    var D = o[1];
    var A = o[2];
    var coo = new long[N];
    var hp = new long[N];
    for (int i = 0; i < N; ++i) {
      var oo = Longs;
      coo[i] = oo[0];
      hp[i] = oo[1];
    }
    Array.Sort (coo, hp);
    long ans = 0;
    int left = 0;
    while (left < N) {
      long last = left;
      long atk = (hp[left] + A - 1) / A;
      ans += atk;
      while (left < N && coo[left] - coo[last] <= 2 * D && hp[left] - atk * A <= 0) {
        ++left;
      }
      if (left < N && coo[left] - coo[last] <= 2 * D)
        hp[left] -= A * atk;

    }
    Put (ans);
  }
}

public class BIT {
  long[] bit;
  int size;
  public BIT (int N) {
    bit = new long[N + 1];
    size = N;
  }
  public BIT (int N, IList<long> list) {
    bit = new long[N + 1];
    size = N;
    for (int i = 0; i < N; ++i) {
      Add (i + 1, list[i]);
    }
  }
  //[1,i]
  public long Sum (int i) {
    long ret = 0;
    for (int x = i; x > 0; x -= x & -x) {
      ret += bit[x];
    }
    return ret;
  }
  //1indexed,[i,j]
  public long Sum (int i, int j) {
    return Sum (j) - Sum (i - 1);
  }
  //1indexed
  public void Add (int i, long x) {
    for (int y = i; y <= size; y += y & -y) {
      bit[y] += x;
    }
  }

}

public class BinarySearch {
  public static int Lower_Bound<T> (IList<T> Array, T Target) {
    return Lower_Bound<T> (Array, Target, Comparer<T>.Default.Compare);
  }
  public static int Lower_Bound<T> (IList<T> Array, T Target, Comparison<T> Comp) {
    var cmp = Comparer<T>.Create (Comp);
    var l = -1; //always ng
    var r = Array.Count (); //always ok
    while (r - l > 1) {
      var mid = l + (r - l) / 2;
      var res = cmp.Compare (Array[mid], Target);
      if (res >= 0) r = mid;
      else l = mid;
    }
    return r;
  }
  public static int Upper_Bound<T> (IList<T> Array, T Target) {
    return Upper_Bound<T> (Array, Target, Comparer<T>.Default.Compare);
  }
  public static int Upper_Bound<T> (IList<T> Array, T Target, Comparison<T> Comp) {
    var cmp = Comparer<T>.Create (Comp);
    var l = -1;
    var r = Array.Count;
    while (r - l > 1) {
      var mid = l + (r - l) / 2;
      var res = cmp.Compare (Array[mid], Target);
      if (res > 0) r = mid;
      else l = mid;
    }
    return r;
  }
}

public static class Math2 {
  public const int INF = 1 << 29;
  public const long INFL = 1L << 60;
  public const long MOD = 1000000007;
  public const int MOD2 = 998244353;

  public static long Power (long a, long b, long MOD = 1000000007) //i^N
  {
    long res = 1;
    while (b > 0) {
      if ((b & 1) != 0) res = res * a % MOD;
      a = a * a % MOD;
      b >>= 1;
    }
    return res;
  }
  public static long Power2 (long a, long b) //i^N
  {
    long res = 1;
    while (b > 0) {
      if ((b & 1) != 0) res = res * a;
      a = a * a;
      b >>= 1;
    }
    return res;
  }
  public static long GCD (long a, long b) {
    while (b > 0) {
      var r = a % b;
      a = b;
      b = r;
    }
    return a;
  }
  public static long LCM (long a, long b) => (b / GCD (a, b)) * a;
  public static long Comb (long n, long r, int MOD = 1000000007) {
    if (r > n - r) r = n - r;
    long Nume = 1;
    long Deno = 1;

    if (r > n - r) r = n - r;
    for (long i = 1; i <= r; ++i) {
      Deno = (Deno * i) % MOD;
      Nume = Nume * (n - i + 1) % MOD;
    }
    return (Nume * inv (Deno)) % MOD;
  }
  public static long Comb2 (long n, long r) {
    long Nume = 1;
    long Deno = 1;

    if (r > n - r) r = n - r;
    for (long i = 1; i <= r; ++i) {
      Deno *= i;
      Nume *= n - i + 1;
    }
    return Nume / Deno;
  }
  public static long inv (long x, int MOD = 1000000007) {
    return Power (x, MOD - 2, MOD);
  }
}
public static class Utils {
  public static void Swap<T> (ref T A, ref T B) {
    T x = A;
    A = B;
    B = x;
  }
  public static int DigitSum (int N) {
    string s = N.ToString ();
    int ret = 0;
    for (int i = 0; i < s.Length; ++i) ret += s[i] - '0';
    return ret;
  }
  public static string ConvertBase (long N, int K) {
    StringBuilder ret = new StringBuilder ();
    while (N > 0) {
      var r = N % K;
      N /= K;
      ret.Append (r);
    }
    return new string (ret.ToString ().ToCharArray ().Reverse ().ToArray ());
  }
}
class Program {
  static void Main (string[] args) {
    var CP = new CP ();
    CP.Solve ();
  }
}
public static class io {
  public static string Str => Console.ReadLine ();
  public static string[] Strs => Str.Split (' ');
  public static long[] Longs => Strs.Select (long.Parse).ToArray ();
  public static int[] Ints => Strs.Select (int.Parse).ToArray ();
  public static char[] Chars => Str.ToArray ();
  public static double[] Doubles => Strs.Select (double.Parse).ToArray ();
  public static long Long1 => Longs[0];
  public static int Int1 => Ints[0];
  public static char Char1 => Chars[0];
  public static double Double1 => Doubles[0];
  public static long[] VerticalRead (int N) { long[] A = new long[N]; for (int i = 0; i < N; ++i) A[i] = Long1; return A; }
  public static void Put (string a) => Console.WriteLine (a);
  public static void Put (params object[] i) => Put (string.Join (" ", i));
  public static void Put<T> (IEnumerable<T> a) => Put (string.Join (" ", a));
  public static void VerticalPut<T> (IEnumerable<T> a) { foreach (var z in a) Put (z); }
  public static void YN (bool i) {
    if (i) Put ("Yes");
    else Put ("No");
  }
}