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: H, N = map(int, input().split())
  AB = []
  
  for i in range(N):
      a, b = map(int, input().split())
      AB.append([a, b])
  INF = 10**15
  dp = [[INF] * (2 * 10**4 + 1) for i in range(N + 1)]
  for i in range(N + 1):
      dp[i][0] = 0
  
  for i in range(N):
      for j in range(1, 2 * 10**4 + 1):
          dp[i + 1][j] = min(dp[i][j], dp[i + 1][j - AB[i][0]] + AB[i][1])
  
  ans = float("inf")
  for i in range(H, 2 * 10**4 + 1):
      ans = min(ans, dp[N][i])
  print(ans)
  
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."}]