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 Rust (1.15.1) (judge verdict: Accepted)

#[allow(dead_code)]
fn is_whitespace(c: &char) -> bool {
    c.is_whitespace()
}

#[allow(dead_code)]
fn is_eol(c: &char) -> bool {
    ['\r', '\n'].contains(c)
}

#[allow(unused_macros)]
macro_rules! input {
    ($var:ident, $($r:tt)*) => {
        input_inner!{$var, $($r)*}
    };

    ($($r:tt)*) => {
        let stdin = std::io::stdin();
        let mut bytes = std::io::Read::bytes(std::io::BufReader::new(stdin.lock()));
        #[allow(unused_mut, unused_variables, bare_trait_objects)]
        let mut next = move |f: &Fn(&char)->bool| -> String{
            bytes
                .by_ref()
                .map(|r|r.unwrap() as char)
                .skip_while(|c|f(&c))
                .take_while(|c|!f(&c))
                .collect()
        };
        input_inner!{next, $($r)*};
    };
}

#[allow(unused_macros)]
macro_rules! input_inner {
    ($next:expr) => {};
    ($next:expr, ) => {};

    ($next:expr, $var:ident : $t:tt $($r:tt)*) => {
        let $var = read_value!($next, $t);
        input_inner!{$next $($r)*}
    };

    ($next:expr, mut $var:ident : $t:tt $($r:tt)*) => {
        let mut $var = read_value!($next, $t);
        input_inner!{$next $($r)*}
    };
}

#[allow(unused_macros)]
macro_rules! read_value {
    ($next:expr, ( $($t:tt),* )) => {
        ( $(read_value!($next, $t)),* )
    };

    ($next:expr, [ $t:tt ; eol ]) => {
        $next(&is_eol).split(' ').map(|x| x.parse::<$t>().unwrap()).collect::<Vec<_>>()
    };

    ($next:expr, [ $t:tt ; $len:expr ]) => {
        (0..$len).map(|_| read_value!($next, $t)).collect::<Vec<_>>()
    };

    ($next:expr, chars) => {
        read_value!($next, String).chars().collect::<Vec<char>>()
    };

    ($next:expr, bytes) => {
        read_value!($next, String).into_bytes()
    };

    ($next:expr, usize1) => {
        read_value!($next, usize) - 1
    };

    ($next:expr, parser) => {
        $next
    };

    ($next:expr, $t:ty) => {
        $next(&is_whitespace).parse::<$t>().expect("Parse error")
    };
}

#[allow(unused_imports)]
use std::cmp::{max, min};
#[allow(unused_imports)]
use std::collections::{BinaryHeap, HashMap, HashSet};

#[allow(dead_code)]
struct Edge {
    from: usize,
    to: usize,
}

#[allow(dead_code)]
type Graph = Vec<Vec<usize>>;
#[allow(dead_code)]
const M: usize = 1_000_000_007;

struct SegmentTree {
    n: usize,
    data: Vec<i64>,
}

fn gcd(a: usize, b: usize) -> usize {
    if b != 0 {
        gcd(b, a % b)
    } else {
        a
    }
}

impl SegmentTree {
    fn f(&self, a: i64, b: i64) -> i64 {
        a + b
    }

    pub fn initial() -> i64 {
        0
    }

    fn new(size: usize) -> Self {
        let n = {
            let mut res = 1;
            while res < size {
                res <<= 1
            }
            res
        };
        let data = vec![Self::initial(); n * 2 - 1];
        SegmentTree { n: n, data: data }
    }

    fn update(&mut self, k: usize, a: i64) {
        let mut index = k + self.n - 1;
        self.data[index] = a;
        while index > 0 {
            index = (index - 1) / 2;
            self.data[index] = self.f(self.data[index * 2 + 1], self.data[index * 2 + 2]);
        }
    }

    // [a, b)
    fn query(&mut self, a: usize, b: usize) -> i64 {
        let (l, r) = (0, self.n);
        self.query_iter(a, b, 0, l, r)
    }

    fn query_iter(&mut self, a: usize, b: usize, k: usize, l: usize, r: usize) -> i64 {
        if r <= a || b <= l {
            Self::initial()
        } else if a <= l && r <= b {
            self.data[k]
        } else {
            let vl = self.query_iter(a, b, k * 2 + 1, l, (l + r) / 2);
            let vr = self.query_iter(a, b, k * 2 + 2, (l + r) / 2, r);
            self.f(vl, vr)
        }
    }
}

fn main() {
    input! {
        n: usize, d: usize, a: usize,
        mut xh: [(usize, usize); n],
    }

    xh.sort();
    let mut s = vec![0; n];
    let mut sum = 0;

    let mut cnt = 0;
    for i in 0..n {
        if xh[i].1 > cnt * a - sum {
            let left = xh[i].1 - (cnt * a - sum);
            let add_cnt = (left + (a - 1)) / a;
            let mut ok = -1;
            let mut ng = n as i64;
            while (ok - ng).abs() > 1 {
                let mid = ((ok + ng) / 2) as usize;
                if xh[mid].0 <= xh[i].0 + d * 2 {
                    ok = mid as i64;
                } else {
                    ng = mid as i64;
                }
            }
            let ok = ok as usize;
            s[ok] += add_cnt * a;
            cnt += add_cnt;
        }
        sum += s[i];
    }
    println!("{}", cnt);
}