Problem:
Score : 600 points

Problem Statement

Snuke, a water strider, lives in a rectangular pond that can be seen as a grid with H east-west rows and W north-south columns. Let (i,j) be the square at the i-th row from the north and j-th column from the west.

Some of the squares have a lotus leaf on it and cannot be entered.
The square (i,j) has a lotus leaf on it if c_{ij} is @, and it does not if c_{ij} is ..

In one stroke, Snuke can move between 1 and K squares (inclusive) toward one of the four directions: north, east, south, and west.
The move may not pass through a square with a lotus leaf. Moving to such a square or out of the pond is also forbidden.

Find the minimum number of strokes Snuke takes to travel from the square (x_1,y_1) to (x_2,y_2).
If the travel from (x_1,y_1) to (x_2,y_2) is impossible, point out that fact.

Constraints

1 \leq H,W,K \leq 10^6

H \times W \leq 10^6

1 \leq x_1,x_2 \leq H

1 \leq y_1,y_2 \leq W

x_1 \neq x_2 or y_1 \neq y_2.

c_{i,j} is . or @.

c_{x_1,y_1} = .

c_{x_2,y_2} = .

All numbers in input are integers.

Input

Input is given from Standard Input in the following format:

H W K
x_1 y_1 x_2 y_2
c_{1,1}c_{1,2} .. c_{1,W}
c_{2,1}c_{2,2} .. c_{2,W}
:
c_{H,1}c_{H,2} .. c_{H,W}

Output

Print the minimum number of strokes Snuke takes to travel from the square (x_1,y_1) to (x_2,y_2), or print -1 if the travel is impossible.

Sample Input 1

3 5 2
3 2 3 4
.....
.@..@
..@..

Sample Output 1

5

Initially, Snuke is at the square (3,2).
He can reach the square (3, 4) by making five strokes as follows:

From (3, 2), go west one square to (3, 1).

From (3, 1), go north two squares to (1, 1).

From (1, 1), go east two squares to (1, 3).

From (1, 3), go east one square to (1, 4).

From (1, 4), go south two squares to (3, 4).

Sample Input 2

1 6 4
1 1 1 6
......

Sample Output 2

2

Sample Input 3

3 3 1
2 1 2 3
.@.
.@.
.@.

Sample Output 3

-1

Solution in Rust (1.42.0) (judge verdict: Wrong Answer)

#[allow(unused_imports)]
use proconio::{input, marker::*};

#[allow(unused_imports)]
use std::collections::{BTreeMap, BTreeSet, BinaryHeap, HashMap, HashSet, VecDeque};

#[allow(unused_imports)]
use std::io::Write;

#[allow(unused_macros)]
macro_rules! debug {
    ($($a:expr),*) => {
        #[cfg(debug_assertions)]
        writeln!(&mut std::io::stderr(), concat!("[DEBUG] ", $(stringify!($a), "={:?} "),*), $($a),*).unwrap();
    }
}

pub trait Monoid<T>: SemiGroup<T> {
    fn identity() -> T;
}
pub trait SemiGroup<T>: Magma<T> {}
pub trait Magma<T> {
    fn operate(lhs: T, rhs: T) -> T;
}
#[derive(Debug, Clone)]
struct SegmentTree<T, U> {
    len: usize,
    nodes: Vec<T>,
    phantom: std::marker::PhantomData<U>,
}
impl<T, U> SegmentTree<T, U>
where
    U: Monoid<T>,
    T: Copy + Clone + std::fmt::Debug,
{
    #[allow(dead_code)]
    fn new(n: usize) -> Self {
        let mut len = 1;
        while len < n {
            len *= 2;
        }
        SegmentTree {
            len: len,
            nodes: vec![U::identity(); 2 * len - 1],
            phantom: Default::default(),
        }
    }
    #[allow(dead_code)]
    fn from_vec(vec: &Vec<T>) -> Self {
        let mut len = 1;
        while len < vec.len() {
            len *= 2;
        }
        let mut nodes = vec![U::identity(); 2 * len - 1];
        for i in 0..vec.len() {
            nodes[i + len - 1] = vec[i];
        }
        for i in (0..len - 1).rev() {
            nodes[i] = U::operate(nodes[2 * i + 1], nodes[2 * i + 2]);
        }
        SegmentTree {
            len: len,
            nodes: nodes,
            phantom: Default::default(),
        }
    }
    #[allow(dead_code)]
    fn update(&mut self, index: usize, val: T) {
        let mut index = index;
        index += self.len - 1;
        self.nodes[index] = val;
        while index > 0 {
            index = (index - 1) / 2;
            self.nodes[index] = U::operate(self.nodes[2 * index + 1], self.nodes[2 * index + 2]);
        }
    }
    fn get(&self, l: usize, r: usize) -> T {
        self._get(l, r, 0, 0, self.len)
    }
    fn _get(&self, ql: usize, qr: usize, index: usize, l: usize, r: usize) -> T {
        if r <= ql || qr <= l {
            return U::identity();
        }
        if ql <= l && r <= qr {
            return self.nodes[index];
        }
        let vl = self._get(ql, qr, 2 * index + 1, l, (l + r) / 2);
        let vr = self._get(ql, qr, 2 * index + 2, (l + r) / 2, r);
        U::operate(vl, vr)
    }
}

#[derive(Clone)]
struct Min;
impl Monoid<usize> for Min {
    fn identity() -> usize {
        std::usize::MAX
    }
}
impl Magma<usize> for Min {
    fn operate(lhs: usize, rhs: usize) -> usize {
        std::cmp::min(lhs, rhs)
    }
}
impl SemiGroup<usize> for Min {}

#[derive(Clone)]
struct Max;
impl Monoid<usize> for Max {
    fn identity() -> usize {
        0
    }
}
impl Magma<usize> for Max {
    fn operate(lhs: usize, rhs: usize) -> usize {
        std::cmp::max(lhs, rhs)
    }
}
impl SemiGroup<usize> for Max {}

fn main() {
    input! {
        h: usize,
        w: usize,
        k: isize,
        xy1: (Usize1,Usize1),
        xy2: (Usize1,Usize1),
        c: [Chars;h],
    }

    let mut row_opt = vec![SegmentTree::<usize, Min>::new(w); h];
    let mut col_opt = vec![SegmentTree::<usize, Min>::new(h); w];

    let mut row_obs_min = vec![SegmentTree::<usize, Min>::new(w + 2); h];
    let mut col_obs_min = vec![SegmentTree::<usize, Min>::new(h + 2); w];

    let mut row_obs_max = vec![SegmentTree::<usize, Max>::new(w + 2); h];
    let mut col_obs_max = vec![SegmentTree::<usize, Max>::new(h + 2); w];

    let mut queue = VecDeque::new();
    let mut visited = vec![vec![false; w]; h];
    queue.push_back(xy1);
    visited[xy1.0][xy1.1] = true;
    row_opt[xy1.0].update(xy1.1, 0);
    col_opt[xy1.1].update(xy1.0, 0);

    for x in 0..h {
        for y in 0..w {
            if c[x][y] == '@' {
                row_obs_min[x].update(y + 1, y + 1);
                col_obs_min[y].update(x + 1, x + 1);
                row_obs_max[x].update(y + 1, y + 1);
                col_obs_max[y].update(x + 1, x + 1);
            }
        }
    }

    use std::cmp::{max, min};

    let dx = [0, 0, 1, -1];
    let dy = [1, -1, 0, 0];
    while let Some((x, y)) = queue.pop_front() {
        let up = max(x as isize - k + 1, 0) as usize;
        let down = min(x + k as usize + 1, h + 1);
        let left = max(y as isize - k + 1, 0) as usize;
        let right = min(y + k as usize + 1, w + 1);

        let left = max(row_obs_max[x].get(left, y + 1) + 1, left);
        let right = min(row_obs_min[x].get(y + 2, right + 1) - 1, right);
        let up = max(col_obs_max[y].get(up, x + 1) + 1, up);
        let down = min(col_obs_min[y].get(x + 2, down + 1) - 1, down);

        let mn = min(
            row_opt[x].get(left - 1, right),
            col_opt[y].get(up - 1, down),
        );

        let tmp = row_opt[x].get(y, y + 1);
        let val = min(tmp, mn.saturating_add(1));
        row_opt[x].update(y, val);
        col_opt[y].update(x, val);

        for i in 0..4 {
            let nx = x as isize + dx[i];
            let ny = y as isize + dy[i];
            if nx < 0 || ny < 0 || nx as usize >= h || ny as usize >= w {
                continue;
            }
            let nx = nx as usize;
            let ny = ny as usize;
            if c[nx][ny] == '@' || visited[nx][ny] {
                continue;
            }
            queue.push_back((nx, ny));
            visited[nx][ny] = true;
        }
    }

    let (x, y) = xy2;
    let val = row_opt[x].get(y, y + 1);
    if val == std::usize::MAX {
        println!("-1");
    } else {
        println!("{}", val);
    }
}