output_description: Print the minimum number of bombs needed to win.
  
  * * *
input_description: Input is given from Standard Input in the following format:
  
      
      
      N D A
      X_1 H_1
      :
      X_N H_N
attempt: from bisect import *
  
  n, d, a, *t = map(int, open(0).read().split())
  c = [0] * -~n
  z = sorted(zip(*[iter(t)] * 2))
  s = 0
  for i, (x, h) in enumerate(z):
      c[i] += c[i - 1]
      h -= c[i]
      t = 0 - -max(0, h) // a
      c[i] += t * a
      c[bisect(z, (x + d + d, 10e9))] -= t * a
      s += t
  print(s)
  
problem_description: 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.
samples: [{"input": "3 3 2\n    1 2\n    5 4\n    9 2", "output": "2\n    \n\nFirst, let us use a bomb at the coordinate 4 to decrease the first and second\nmonsters' health by 2.\n\nThen, use a bomb at the coordinate 6 to decrease the second and third\nmonsters' health by 2.\n\nNow, all the monsters' healths are 0. We cannot make all the monsters' health\ndrop to 0 or below with just one bomb.\n\n* * *"}, {"input": "9 4 1\n    1 5\n    2 4\n    3 3\n    4 2\n    5 1\n    6 2\n    7 3\n    8 4\n    9 5", "output": "5\n    \n\nWe should use five bombs at the coordinate 5.\n\n* * *"}, {"input": "3 0 1\n    300000000 1000000000\n    100000000 1000000000\n    200000000 1000000000", "output": "3000000000\n    \n\nWatch out for overflow."}]