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.*;
import java.util.*;
import java.lang.*;

public class Main {
	static int n,m;
	static char[][] grid;
	public static void main(String[] args) {
		FastScanner in = new FastScanner();
//		PrintWriter out = new PrintWriter(System.out);
		n = in.nextInt();
		m = in.nextInt();
		int startx = in.nextInt()-1, starty = in.nextInt()-1;
		int destx = in.nextInt()-1, desty = in.nextInt()-1;
		grid = new char[n][m];
		for(int i=0;i<n;i++) {
			char[] line = in.next().toCharArray();	
			for(int j = 0;j<m;j++) 
				grid[i][j] = line[j];
		}
		ArrayDeque<Integer> x = new ArrayDeque<Integer>(),y = new ArrayDeque<>();
		x.add(startx);
		y.add(starty);
		int[][] cost = new int[n][m];
		for(int i=0;i<n;i++) Arrays.fill(cost[i], 1_000_000_000);
		cost[startx][starty] = 0;
		int[] dx = {0,0,-1,1};
		int[] dy = {1,-1,0,0};
		while(!x.isEmpty()) {
			int curx = x.poll();
			int cury = y.poll();
			for(int i=0;i<dx.length;i++) {
				int nx = curx+dx[i];
				int ny = cury+dy[i];
				if(!legal(nx,ny)) continue;
				if(cost[nx][ny]>cost[curx][cury]) {
					cost[nx][ny] = cost[curx][cury];
					x.addFirst(nx);
					y.addFirst(ny);
				}
			}
			for(int nx=curx-2;nx<curx+3;nx++) {
				for(int ny=cury-2;ny<cury+3;ny++)
				{
					if(!legal(nx,ny)) continue;
					if(cost[nx][ny]>cost[curx][cury]+1) {
						cost[nx][ny] = cost[curx][cury]+1;
						x.addLast(nx);
						y.addLast(ny);
					}
				}
			}
		}
		System.out.println(cost[destx][desty]>(int)1e7?-1:cost[destx][desty]);
//		out.close();
	}
	public static boolean legal(int x,int y) {
		if(x<0||x>=n||y<0||y>=m||grid[x][y]=='#') return false;
		return true;
	}
	static final Random random=new Random();
	//	static void ruffleSort(Pair[] a) {
	//		int n=a.length;//shuffle, then sort 
	//		for (int i=0; i<n; i++) {
	//			int oi=random.nextInt(n);
	//			Pair temp=a[oi];
	//			a[oi]=a[i]; a[i]=temp;
	//		}
	//		Arrays.sort(a);
	//	}
	static void ruffleSort(int[] a) {
		int n=a.length;//shuffle, then sort 
		for (int i=0; i<n; i++) {
			int oi=random.nextInt(n), temp=a[oi];
			a[oi]=a[i]; a[i]=temp;
		}
		Arrays.sort(a);
	}
	static void ruffleSort(char[] a) {
		int n=a.length;//shuffle, then sort 
		for (int i=0; i<n; i++) {
			int oi=random.nextInt(n);
			char temp=a[oi];
			a[oi]=a[i]; a[i]=temp;
		}
		Arrays.sort(a);
	}
	static class FastScanner {
		BufferedReader br=new BufferedReader(new InputStreamReader(System.in));
		StringTokenizer st=new StringTokenizer("");
		String next() {
			while (!st.hasMoreTokens())
				try {
					st=new StringTokenizer(br.readLine());
				} catch (IOException e) {
					e.printStackTrace();
				}
			return st.nextToken();
		}
		
		int nextInt() {
			return Integer.parseInt(next());
		}
		int[] readArray(int n) {
			int[] a=new int[n];
			for (int i=0; i<n; i++) a[i]=nextInt();
			return a;
		}
		long nextLong() {
			return Long.parseLong(next());
		}
	}
}

//class Pair implements Comparable<Pair>{
//    int a;
//    int b;
//    public Pair(int a, int b) {
//        this.a = a;
//        this.b = b;
//    }
//    public int compareTo(Pair o) {
//        if(this.a==o.a)
//            return this.b - o.b;
//        return this.a - o.a;
//    }   
//}