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 Java (OpenJDK 1.8.0) (judge verdict: Accepted)

import java.io.OutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.PrintWriter;
import java.util.Arrays;
import java.util.Collection;
import java.util.function.IntBinaryOperator;
import java.io.IOException;
import java.io.InputStreamReader;
import java.util.TreeSet;
import java.util.StringTokenizer;
import java.util.Queue;
import java.io.BufferedReader;
import java.util.ArrayDeque;
import java.io.InputStream;

/**
 * Built using CHelper plug-in
 * Actual solution is at the top
 *
 * @author AnandOza
 */
public class Main {
    public static void main(String[] args) {
        InputStream inputStream = System.in;
        OutputStream outputStream = System.out;
        InputReader in = new InputReader(inputStream);
        PrintWriter out = new PrintWriter(outputStream);
        FPondSkater solver = new FPondSkater();
        solver.solve(1, in, out);
        out.close();
    }

    static class FPondSkater {
        public void solve(int testNumber, InputReader in, PrintWriter out) {
            int h = in.nextInt(), w = in.nextInt(), k = in.nextInt();
            int x1 = in.nextInt(), y1 = in.nextInt();
            int x2 = in.nextInt(), y2 = in.nextInt();

            boolean[][] blocked = new boolean[h + 2][w + 2];
            for (int i = 1; i <= h; i++) {
                char[] row = in.next().toCharArray();
                for (int j = 1; j <= w; j++) {
                    blocked[i][j] = row[j - 1] == '@';
                }
            }
            h += 2;
            w += 2;
            for (int i = 0; i < h; i++) {
                blocked[i][0] = blocked[i][w - 1] = true;
            }
            for (int j = 0; j < w; j++) {
                blocked[0][j] = blocked[h - 1][j] = true;
            }

            int[][] distance = new int[h][w];
            for (int[] row : distance)
                Arrays.fill(row, -1);
            distance[x1][y1] = 0;
            Queue<Pii> q = new ArrayDeque<>();
            q.add(Pii.of(x1, y1));

            boolean[][] horiz = new boolean[h][w];
            boolean[][] vert = new boolean[h][w];
            horiz[x1][y1] = vert[x1][y1] = true;

            TreeSet<Integer>[] rows = new TreeSet[h];
            TreeSet<Integer>[] cols = new TreeSet[w];
            for (int i = 0; i < h; i++) {
                rows[i] = new TreeSet<>();
                for (int j = 0; j <= w; j++) {
                    rows[i].add(j);
                }
            }
            for (int i = 0; i < w; i++) {
                cols[i] = new TreeSet<>();
                for (int j = 0; j <= h; j++) {
                    cols[i].add(j);
                }
            }

            while (!q.isEmpty()) {
                Pii cur = q.poll();

                int d = distance[cur.first][cur.second];

                IntBinaryOperator process = (x, y) -> {
                    distance[x][y] = d + 1;
                    rows[x].remove(y);
                    cols[y].remove(x);
                    q.add(Pii.of(x, y));
                    return 0;
                };

                for (int x = cur.first + 1, y = cur.second; Math.abs(cur.first - x) <= k; x = cols[y].higher(x)) {
                    if (blocked[x][y])
                        break;

                    if (distance[x][y] != -1)
                        continue;

                    process.applyAsInt(x, y);
                }
                for (int x = cur.first - 1, y = cur.second; Math.abs(cur.first - x) <= k; x = cols[y].lower(x)) {
                    if (blocked[x][y])
                        break;

                    if (distance[x][y] != -1)
                        continue;

                    process.applyAsInt(x, y);
                }
                for (int x = cur.first, y = cur.second + 1; Math.abs(cur.second - y) <= k; y = rows[x].higher(y)) {
                    if (blocked[x][y])
                        break;

                    if (distance[x][y] != -1)
                        continue;

                    process.applyAsInt(x, y);
                }
                for (int x = cur.first, y = cur.second - 1; Math.abs(cur.second - y) <= k; y = rows[x].lower(y)) {
                    if (blocked[x][y])
                        break;

                    if (distance[x][y] != -1)
                        continue;

                    process.applyAsInt(x, y);
                }
            }

            out.println(distance[x2][y2]);
        }

    }

    static class InputReader {
        public BufferedReader reader;
        public StringTokenizer tokenizer;

        public InputReader(InputStream stream) {
            reader = new BufferedReader(new InputStreamReader(stream), 32768);
            tokenizer = null;
        }

        public String next() {
            while (tokenizer == null || !tokenizer.hasMoreTokens()) {
                try {
                    tokenizer = new StringTokenizer(reader.readLine());
                } catch (IOException e) {
                    throw new RuntimeException(e);
                }
            }
            return tokenizer.nextToken();
        }

        public int nextInt() {
            return Integer.parseInt(next());
        }

    }

    static class Pii {
        public final int first;
        public final int second;

        public Pii(int first, int second) {
            this.first = first;
            this.second = second;
        }

        public static Pii of(int first, int second) {
            return new Pii(first, second);
        }

        public boolean equals(Object o) {
            if (this == o)
                return true;
            if (o == null || getClass() != o.getClass())
                return false;
            Pii pair = (Pii) o;
            return first == pair.first && second == pair.second;
        }

        public int hashCode() {
            return Arrays.hashCode(new int[]{first, second});
        }

        public String toString() {
            return "(" + first + ", " + second + ')';
        }

    }
}