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 Java8 (OpenJDK 1.8.0) (judge verdict: Accepted)

import java.io.*;
import java.util.*;
//import java.math.*;
//import java.awt.Point;
 
public class Main {
	//static final long MOD = 998244353L;
	//static final long INF = -1000000000000000007L;
	static final long MOD = 1000000007L;
	//static final int INF = 1000000007;
	
	public static void main(String[] args) {
		FastScanner sc = new FastScanner();
		PrintWriter pw = new PrintWriter(System.out);
		int N = sc.ni();
		int D = sc.ni();
		int A = sc.ni();
		long[][] pos = new long[N][2];
		for (int i = 0; i < N; i++) {
			pos[i] = new long[]{sc.nl(),sc.nl()};
		}
		pos = sort(pos);
		SegmentTree health = new SegmentTree(N);
		for (int i = 0; i < N; i++) {
			health.add(1, 1, N, i+1, i+1, pos[i][1]);
		}
		long ans = 0;
		for (int i = 0; i < N; i++) {
			long H = health.minQuery(1, 1, N, i+1, i+1);
			if (H <= 0) {
				//this monster is already dead.
				continue;
			}
			long xPos = pos[i][0]+D; //this is where you should throw the bomb. to get the maximum range
			long num = (H+A-1)/A; //number of bombs to throw
			long damage = (-1)*num*A; //damage of the bombs
			//binary search for the last monster hit by the bomb
			int low = i;
			int high = N-1;
			while (low < high) {
				int med = (low+high+1)/2;
				if (pos[med][0] <= xPos+D) {
					low = med;
				} else {
					high = med-1;
				}
			}
			//everyone from [i..last] is hit by the bombs.
			ans += num;
			health.add(1, 1, N, i+1, low+1, damage);
		}
		pw.println(ans);
		pw.close();
	}
	
	static class SegmentTree {

		public long[][] ranges;

		public SegmentTree(int N) {
			ranges = new long[4*N+1][4];
		}

		//range update nums[updateL..updateR] += val;
		public void add(int n, int L, int R, int updateL, int updateR, long val) {
			push(n,L,R);
			if (updateL <= L && R <= updateR) {
				//fully contained
				ranges[n][3] += val;
				push(n,L,R);
				return;
			} else if (L>updateR || R<updateL || L==R) {
				//not contained at all or leaf
				return;
			}
			add(2*n,L,(L+R)/2,updateL,updateR,val); 
			add(2*n+1,(L+R)/2+1,R,updateL,updateR,val);
			ranges[n][1] = Math.min(ranges[2*n][1],ranges[2*n+1][1]);
			ranges[n][2] = Math.max(ranges[2*n][2],ranges[2*n+1][2]);
			ranges[n][0] = ranges[2*n][0] + ranges[2*n+1][0];
		}

		public long minQuery(int n, int L, int R, int Lq, int Rq) {
			push(n,L,R);
			if (Lq <= L && R <= Rq) {
				return ranges[n][1];
			} else if (R < Lq || Rq < L || L==R) {
				return 1000000000000007L;
			} else {
				return Math.min(minQuery(2*n,L,(L+R)/2,Lq,Rq),minQuery(2*n+1,(L+R)/2+1,R,Lq,Rq));
			}
		}

		public long maxQuery(int n, int L, int R, int Lq, int Rq) {
			push(n,L,R);
			if (Lq <= L && R <= Rq) {
				return ranges[n][2];
			} else if (R < Lq || Rq < L || L==R) {
				return -1L;
			} else {
				return Math.max(maxQuery(2*n,L,(L+R)/2,Lq,Rq),maxQuery(2*n+1,(L+R)/2+1,R,Lq,Rq));
			}
		}

		public long sumQuery(int n, int L, int R, int Lq, int Rq) {
			push(n,L,R);
			if (Lq <= L && R <= Rq) {
				return ranges[n][0];
			} else if (R < Lq || Rq < L || L==R) {
				return 0L;
			} else {
				return (sumQuery(2*n,L,(L+R)/2,Lq,Rq) + sumQuery(2*n+1,(L+R)/2+1,R,Lq,Rq));
			}
		}

		public void push(int n, int L, int R) {
			ranges[n][1] += ranges[n][3];
			ranges[n][2] += ranges[n][3];
			ranges[n][0] += (R-L+1)*ranges[n][3];

			if (L < R) {
				ranges[2*n][3] += ranges[n][3];
				ranges[2*n+1][3] += ranges[n][3];
			}

			ranges[n][3] = 0;
		}
	}
	
	public static long dist(long[] p1, long[] p2) {
		return (Math.abs(p2[0]-p1[0])+Math.abs(p2[1]-p1[1]));
	}
	
	//Find the GCD of two numbers
	public static long gcd(long a, long b) {
		if (a < b) return gcd(b,a);
		if (b == 0)
			return a;
		else
			return gcd(b,a%b);
	}
	
	//Fast exponentiation (x^y mod m)
	public static long power(long x, long y, long m) { 
		long ans = 1;
		x %= m;
		while (y > 0) { 
			if(y % 2 == 1) 
				ans = (ans * x) % m; 
			y /= 2;  
			x = (x * x) % m;
		} 
		return ans; 
	}
	
	public static int[][] shuffle(int[][] array) {
		Random rgen = new Random();
		for (int i = 0; i < array.length; i++) {
		    int randomPosition = rgen.nextInt(array.length);
		    int[] temp = array[i];
		    array[i] = array[randomPosition];
		    array[randomPosition] = temp;
		}
		return array;
	}
	
    public static int[][] sort(int[][] array) {
    	//Sort an array (immune to quicksort TLE)
 
		Arrays.sort(array, new Comparator<int[]>() {
			  @Override
        	  public int compare(int[] arr1, int[] arr2) {
				  //return arr1[0]-arr2[0]; //ascending order
				  if (arr1[0] != arr2[0]) {
					  return arr1[0]-arr2[0];
				  } else {
					  return arr1[1] - arr2[1];
				  }
	          }
		});
		return array;
	}
    
    public static long[][] sort(long[][] array) {
    	//Sort an array (immune to quicksort TLE)
		Random rgen = new Random();
		for (int i = 0; i < array.length; i++) {
		    int randomPosition = rgen.nextInt(array.length);
		    long[] temp = array[i];
		    array[i] = array[randomPosition];
		    array[randomPosition] = temp;
		}
		Arrays.sort(array, new Comparator<long[]>() {
			  @Override
        	  public int compare(long[] arr1, long[] arr2) {
				  //return 0;
				  if (arr1[0] < arr2[0]) {
					  return -1;
				  } else if (arr1[0] > arr2[0]) {
					  return 1;
				  } else {
					  return 0;
				  }
	          }
		});
		return array;
	}
    
    static class FastScanner { 
        BufferedReader br; 
        StringTokenizer st; 
  
        public FastScanner() { 
            br = new BufferedReader(new InputStreamReader(System.in)); 
        } 
  
        String next() { 
            while (st == null || !st.hasMoreElements()) { 
                try { 
                    st = new StringTokenizer(br.readLine());
                } catch (IOException  e) { 
                    e.printStackTrace(); 
                } 
            } 
            return st.nextToken(); 
        } 
  
        int ni() { 
            return Integer.parseInt(next()); 
        } 
  
        long nl() { 
            return Long.parseLong(next()); 
        } 
  
        double nd() { 
            return Double.parseDouble(next()); 
        } 
  
        String nextLine() { 
            String str = ""; 
            try { 
                str = br.readLine(); 
            } catch (IOException e) {
                e.printStackTrace(); 
            } 
            return str;
        }
    }
}