Problem:
Score : 400 points

Problem Statement

A maze is composed of a grid of H \times W squares - H vertical, W horizontal.

The square at the i-th row from the top and the j-th column from the left - (i,j) - is a wall if S_{ij} is # and a road if S_{ij} is ..

There is a magician in (C_h,C_w). He can do the following two kinds of moves:

Move A: Walk to a road square that is vertically or horizontally adjacent to the square he is currently in.

Move B: Use magic to warp himself to a road square in the 5\times 5 area centered at the square he is currently in.

In either case, he cannot go out of the maze.

At least how many times does he need to use the magic to reach (D_h, D_w)?

Constraints

1 \leq H,W \leq 10^3

1 \leq C_h,D_h \leq H

1 \leq C_w,D_w \leq W

S_{ij} is # or ..

S_{C_h C_w} and S_{D_h D_w} are ..

(C_h,C_w) \neq (D_h,D_w)

Input

Input is given from Standard Input in the following format:

H W
C_h C_w
D_h D_w
S_{11}\ldots S_{1W}
\vdots
S_{H1}\ldots S_{HW}

Output

Print the minimum number of times the magician needs to use the magic. If he cannot reach (D_h,D_w), print -1 instead.

Sample Input 1

4 4
1 1
4 4
..#.
..#.
.#..
.#..

Sample Output 1

1

For example, by walking to (2,2) and then using the magic to travel to (4,4), just one use of magic is enough.

Note that he cannot walk diagonally.

Sample Input 2

4 4
1 4
4 1
.##.
####
####
.##.

Sample Output 2

-1

He cannot move from there.

Sample Input 3

4 4
2 2
3 3
....
....
....
....

Sample Output 3

0

No use of magic is needed.

Sample Input 4

4 5
1 2
2 5
#.###
####.
#..##
#..##

Sample Output 4

2

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.HashSet;
import java.util.StringTokenizer;
import java.util.BitSet;
import java.io.BufferedReader;
import java.util.Queue;
import java.util.ArrayDeque;
import java.io.InputStream;

/**
 * Built using CHelper plug-in
 * Actual solution is at the top
 */
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);
        DWizardInMaze solver = new DWizardInMaze();
        solver.solve(1, in, out);
        out.close();
    }

    static class DWizardInMaze {
        int h;
        int w;
        int n;
        BitSet grid;

        public void solve(int testNumber, InputReader in, PrintWriter out) {
            h = in.nextInt();
            w = in.nextInt();
            n = h * w;
            int start = c(in.nextInt() - 1, in.nextInt() - 1);
            int end = c(in.nextInt() - 1, in.nextInt() - 1);

            grid = new BitSet();
            for (int i = 0, index = 0; i < h; i++) {
                char[] row = in.next().toCharArray();
                for (char c : row) {
                    if (c == '#')
                        grid.set(index);
                    index++;
                }
            }

            UnionFind uf = new UnionFind(n);

            IntBinaryOperator tryMerge = (u, v) -> {
                if (grid.get(u) || grid.get(v))
                    return 0;
                uf.union(u, v);
                return 0;
            };

            for (int i = 0; i < h; i++) {
                for (int j = 0; j < w; j++) {
                    int c = c(i, j);
                    if (i - 1 >= 0)
                        tryMerge.applyAsInt(c, c - w);
                    if (i + 1 < h)
                        tryMerge.applyAsInt(c, c + w);
                    if (j - 1 >= 0)
                        tryMerge.applyAsInt(c, c - 1);
                    if (j + 1 < w)
                        tryMerge.applyAsInt(c, c + 1);
                }
            }

            HashSet<Integer>[] adj = new HashSet[n];
            for (int i = 0; i < n; i++) {
                adj[i] = new HashSet<>();
            }
            for (int i = 0; i < h; i++) {
                for (int j = 0; j < w; j++) {
                    int u = c(i, j);
                    if (grid.get(u))
                        continue;
                    for (int x = i - 2; x <= i + 2; x++) {
                        if (x < 0 || x >= h)
                            continue;
                        for (int y = j - 2; y <= j + 2; y++) {
                            if (y < 0 || y >= w)
                                continue;
                            if (Math.abs(y - j) <= 1 && Math.abs(y - i) <= 1)
                                continue;
                            int v = c(x, y);
                            if (grid.get(v))
                                continue;

                            adj[uf.rep(u)].add(uf.rep(v));
                            adj[uf.rep(v)].add(uf.rep(u));
                        }
                    }
                }
            }
            int[] distance = Graphs.distanceViaBFS(adj, uf.rep(start));

            out.println(distance[uf.rep(end)]);
        }

        private int c(int i, int j) {
            return w * i + j;
        }

    }

    static class UnionFind {
        private final int[] __rep;
        private final int[] __size;

        public UnionFind(int n) {
            __rep = new int[n];
            __size = new int[n];
            for (int i = 0; i < n; i++) {
                __rep[i] = i;
                __size[i] = 1;
            }
        }

        public int rep(int x) {
            if (__rep[x] == x) {
                return x;
            }

            int r = rep(__rep[x]);
            __rep[x] = r;
            return r;
        }

        public int size(int x) {
            return __size[rep(x)];
        }

        public boolean union(int x, int y) {
            x = rep(x);
            y = rep(y);

            if (x == y) {
                return false;
            }

            if (size(x) < size(y)) {
                int t = x;
                x = y;
                y = t;
            }

            // now size(x) >= size(y)

            __rep[y] = x;
            __size[x] += __size[y];
            return true;
        }

    }

    static class InputReader {
        public final 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 Graphs {
        public static int[] distanceViaBFS(Iterable<Integer>[] adj, int start) {
            final int n = adj.length;
            int[] distance = new int[n];
            Arrays.fill(distance, -1);
            Queue<Integer> q = new ArrayDeque<>();
            q.add(start);
            distance[start] = 0;

            while (!q.isEmpty()) {
                int cur = q.poll();
                for (int next : adj[cur]) {
                    if (distance[next] != -1)
                        continue;

                    q.add(next);
                    distance[next] = distance[cur] + 1;
                }
            }

            return distance;
        }

    }
}