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 Haskell (GHC 7.10.3) (judge verdict: Time Limit Exceeded)

import Control.Monad
import Data.List

main = do
  li <- getLine
  let [n,d,a] = map read $ words li
  xhs <- forM [1..n] (\_ -> do
    li <- getLine
    let [x,h] = map read $ words li
    return (x,h)
    )
  let ans = compute n d a xhs
  print ans

compute :: Int -> Int -> Int -> [(Int,Int)] -> Int
compute n d a xhs = loop 0 [] (sort xhs)
  where
    d2 = d * 2
    loop n _ [] = n
    loop n ybs ((x,h):xhs)
      | h1 <= 0 = loop n ybs1 xhs
      | True    = loop (n+k) ((x+d2, a*k):ybs1) xhs
      where
        ybs1 = filter ((x <=).fst) ybs
        h1 = h - sum (map snd ybs1)
        k = (h1 + a - 1) `div` a