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 Ruby (2.3.3) (judge verdict: Runtime Error)

n, d, a = gets.split(" ").map(&:to_i)
monsters = []
num = 0
boms = []
n.times do |i|
  mon = gets.split(" ").map(&:to_i)
  mon[1] = mon[1].fdiv(a).ceil
  monsters[i] = mon
end
monsters = monsters.sort_by {|y| y[0]}
n.times do |i|
  if boms.length != 0
    boms.delete_if do |bom|
      !monsters[i][0].between?(bom[0]-d, bom[0]+d)
    end
  end
  damage_sum = boms.inject(:+) || 0
  if monsters[i][1] > damage_sum
    boms.push([monsters[i][0]+d, monsters[i][1]-damage_sum])
  	num += monsters[i][1]-damage_sum
  end
end
puts num