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 C# (.NET Core 3.1.201) (judge verdict: Accepted)

using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.Diagnostics.Tracing;
using System.Linq;
using System.Runtime.CompilerServices;
using System.Runtime.Intrinsics.X86;
using System.Security.Cryptography;
using System.Text;
using System.Text.RegularExpressions;
using System.Threading;
using Extensions;
using DSA;
using static System.Math;
using static Extensions.MathExtension;
using static Extensions.ConsoleInputExtension;
using static Extensions.ConsoleOutputExtension;

class Solver
{
    public void Solve()
    {
        // checked
        {
            //Solve Code Here
            int h = Cin, w = Cin;
            int ch = Cin - 1, cw = Cin - 1;
            int dh = Cin - 1, dw = Cin - 1;
            var s = new bool[w, h];
            for (int y = 0; y < h; y++)
            {
                string ss = Cin;
                for (int x = 0; x < w; x++) s[x, y] = ss[x] == '.';
            }

            var uf = new UnionFind(h * w);
            for (int y = 0; y < h; y++)
            {
                for (int x = 0; x < w; x++)
                {
                    if (s[x, y])
                    {
                        if (y > 0 && s[x, y - 1]) uf.UnionGroup(y * w + x, (y - 1) * w + x);
                        if (x > 0 && s[x - 1, y]) uf.UnionGroup(y * w + x, y * w + x - 1);
                        if (y < h - 1 && s[x, y + 1]) uf.UnionGroup(y * w + x, (y + 1) * w + x);
                        if (x < w - 1 && s[x + 1, y]) uf.UnionGroup(y * w + x, y * w + x + 1);
                    }
                }
            }

            var visited = new bool[w, h];
            var min = new long[w, h];
            for (int i = 0; i < w; i++)
            for (int j = 0; j < h; j++)
                min[i, j] = long.MaxValue;

            // var pq = new PriorityQueue<(int x, int y, int cost)>();
            var pq = new LinkedList<(int x, int y, int cost)>();
            pq.AddLast((cw, ch, 0));
            min[cw, ch] = 0;
            while (pq.Count > 0)
            {
                var (x, y, c) = pq.First();
                pq.RemoveFirst();
                if (x == dw && y == dh)
                {
                    Coutln(c);
                    return;
                }

                if (c > min[x, y]) continue;
                // Coutln($"{x},{y},{c}");
                visited[x, y] = true;

                if (x > 0 && s[x - 1, y] && !visited[x - 1, y])
                {
                    pq.AddFirst((x - 1, y, c));
                    min[x - 1, y] = Min(min[x - 1, y], c);
                }

                if (y > 0 && s[x, y - 1] && !visited[x, y - 1])
                {
                    pq.AddFirst((x, y - 1, c));
                    min[x, y - 1] = Min(min[x, y - 1], c);
                }

                if (x < w - 1 && s[x + 1, y] && !visited[x + 1, y])
                {
                    pq.AddFirst((x + 1, y, c));
                    min[x + 1, y] = Min(min[x + 1, y], c);
                }

                if (y < h - 1 && s[x, y + 1] && !visited[x, y + 1])
                {
                    pq.AddFirst((x, y + 1, c));
                    min[x, y + 1] = Min(min[x, y + 1], c);
                }


                for (int yy = Max(y - 2, 0); yy <= Min(y + 2, h - 1); yy++)
                for (int xx = Max(x - 2, 0); xx <= Min(x + 2, w - 1); xx++)
                {
                    if (s[xx, yy] && !uf.IsSameGroup(yy * w + xx, y * w + x) && min[xx, yy] > c + 1)
                    {
                        pq.AddLast((xx, yy, c + 1));
                        min[xx, yy] = Min(min[xx, yy], c + 1);
                    }
                }
            }

            Coutln(-1);
        }
    }
}

//Other Classes Here
#if !DEBUG
class EntryPoint
{
    static void Main(string[] args)
    {
        new Solver().Solve();
        Flush();
    }
}

#endif

namespace DSA
{
    public class PriorityQueue<T>
    {
        private readonly List<Tuple<int, T>> _list = new List<Tuple<int, T>>();

        public int Count => _list.Count;

        public void Push(T item, int priority)
        {
            _list.Add(Tuple.Create(priority, item));

            int itemIndex = Count - 1, parentIndex;
            while ((parentIndex = GetParentIndex(itemIndex)) != -1 &&
                   priority > _list[parentIndex].Item1)
            {
                Swap(itemIndex, parentIndex);
                itemIndex = parentIndex;
            }
        }

        private int GetParentIndex(int index)
        {
            if (index == 0) return -1;
            return ((index + 1) >> 1) - 1;
        }

        private Tuple<int, int> GetChildrenIndex(int index)
        {
            var item2 = (index + 1) << 1;
            var item1 = item2 - 1;
            return Tuple.Create(item1, item2);
        }

        public T Pop()
        {
            if (Count <= 0) throw new IndexOutOfRangeException();
            var item = _list[0].Item2;
            _list[0] = _list[Count - 1];
            _list.RemoveAt(Count - 1);

            int index = 0;
            Tuple<int, int> childrenIndex = GetChildrenIndex(index);
            while (childrenIndex.Item1 < Count || childrenIndex.Item2 < Count)
            {
                if (childrenIndex.Item2 >= Count || _list[childrenIndex.Item1].Item1 > _list[childrenIndex.Item2].Item1)
                {
                    if (_list[childrenIndex.Item1].Item1 <= _list[index].Item1) return item;

                    Swap(index, childrenIndex.Item1);
                    index = childrenIndex.Item1;
                }
                else
                {
                    if (_list[childrenIndex.Item2].Item1 <= _list[index].Item1) return item;
                    Swap(index, childrenIndex.Item2);
                    index = childrenIndex.Item2;
                }

                childrenIndex = GetChildrenIndex(index);
            }

            return item;
        }

        public T Peek()
        {
            return _list[0].Item2;
        }

        private void Swap(int index1, int index2)
        {
            var tmp = _list[index1];
            _list[index1] = _list[index2];
            _list[index2] = tmp;
        }
    }

    public class UnionFind
    {
        private readonly int[] _array;

        public UnionFind(int size)
        {
            _array = new int[size];
            for (int i = 0; i < size; i++)
            {
                _array[i] = i;
            }
        }

        public int GetRootNode(int n)
        {
            if (_array[n] == n) return n;
            return _array[n] = GetRootNode(_array[n]);
        }

        public void UnionGroup(int a, int b)
        {
            var rootA = GetRootNode(a);
            var rootB = GetRootNode(b);
            if (rootA == rootB) return;
            _array[rootA] = rootB;
        }

        public bool IsSameGroup(int a, int b) => GetRootNode(a) == GetRootNode(b);

        public bool IsRoot(int n) => _array[n] == n;
    }

    public delegate T SegTreeCombiner<T>(T item1, T item2);

    public class SegTree<T>
    {
        private readonly T _defaultItem;
        private readonly SegTreeCombiner<T> _func;
        private T[] List;
        private int size;

        public SegTree(T[] list, T defaultItem, SegTreeCombiner<T> func)
        {
            _defaultItem = defaultItem;
            _func = func;
            size = 1;
            while (size < list.Length) size <<= 1;
            List = new T[2 * size - 1];
            for (int i = 0; i < list.Length; i++) List[i + size - 1] = list[i];
            for (int i = list.Length; i < size; i++) List[i + size - 1] = defaultItem;
            for (int i = size - 1 - 1; i >= 0; i--)
            {
                List[i] = _func(List[2 * i + 1], List[2 * i + 2]);
            }
        }

        public void Update(int index, T value)
        {
            index += size - 1;
            List[index] = value;
            while (index > 0)
            {
                index = (index - 1) >> 1;
                List[index] = _func(List[2 * index + 1], List[2 * index + 2]);
            }
        }

        public T Query(int a, int b)
        {
            return Query(a, b, 0, 0, size);
        }

        private T Query(int a, int b, int k, int l, int r)
        {
            if (r <= a || b <= l) return _defaultItem;
            if (a <= l && r <= b) return List[k];
            return _func(Query(a, b, k * 2 + 1, l, (l + r) >> 1), Query(a, b, k * 2 + 2, (l + r) >> 1, r));
        }
    }

    public static class BinarySearch
    {
        public delegate bool Terms<T>(T i);

        public static int UpperBound(int initLeft, int initRight, Terms<int> term)
        {
            int left = initLeft - 1, right = initRight;
            while (right - left > 1)
            {
                int mid = (left + right) >> 1;
                if (mid != initLeft - 1 && term(mid)) left = mid;
                else right = mid;
            }

            return left;
        }

        public static long UpperBound(long initLeft, long initRight, Terms<long> term)
        {
            long left = initLeft - 1, right = initRight;
            while (right - left > 1)
            {
                long mid = (left + right) >> 1;
                if (mid != initLeft - 1 && term(mid)) left = mid;
                else right = mid;
            }

            return left;
        }

        public static int LowerBound(int initLeft, int initRight, Terms<int> term)
        {
            int left = initLeft - 1, right = initRight;
            while (right - left > 1)
            {
                int mid = (left + right) >> 1;
                if (mid != initRight && term(mid)) right = mid;
                else left = mid;
            }

            return right;
        }

        public static long LowerBound(long initLeft, long initRight, Terms<long> term)
        {
            long left = initLeft - 1, right = initRight;
            while (right - left > 1)
            {
                long mid = (left + right) >> 1;
                if (term(mid)) right = mid;
                else left = mid;
            }

            return right;
        }
    }
}


struct ModInt
{
    public long Value { get; }
    public static long Mod { get; set; } = 1000000000L + 7;

    public ModInt(long l)
    {
        long value = l % Mod;
        while (value < 0) value += Mod;
        Value = value % Mod;
    }

    public static implicit operator long(ModInt m)
    {
        return m.Value;
    }

    public static implicit operator ModInt(long l)
    {
        return new ModInt(l);
    }

    public static ModInt operator +(ModInt a, ModInt b)
    {
        return new ModInt(a.Value + b.Value);
    }

    public static ModInt operator -(ModInt a, ModInt b)
    {
        return new ModInt(a.Value - b.Value);
    }

    public static ModInt operator *(ModInt a, ModInt b)
    {
        return new ModInt(a.Value * b.Value);
    }

    public static ModInt operator /(ModInt a, ModInt b)
    {
        return new ModInt(a * b.Inverse());
    }

    public override string ToString()
    {
        return Value.ToString();
    }
}

static class ModIntMath
{
    public static ModInt Pow(this ModInt a, ModInt b)
    {
        var pow = b.Value;
        var m = a;
        ModInt ans = new ModInt(1);
        while (pow != 0)
        {
            if ((pow & 1) == 1)
            {
                ans *= m;
            }

            pow >>= 1;
            m *= m;
        }

        return ans;
    }

    public static ModInt Inverse(this ModInt m)
    {
        return m.Pow((ModInt) (ModInt.Mod - 2));
    }
}


namespace Extensions
{
    public class ConsoleInputExtension
    {
        private static readonly ConsoleInputExtension _cin = new ConsoleInputExtension();

        public static ConsoleInputExtension Cin => _cin;

        private static readonly Queue<string> _inputQueue = new Queue<string>();

        private ConsoleInputExtension()
        {
        }

        public static implicit operator string(ConsoleInputExtension _)
        {
            if (_inputQueue.Count == 0)
                Console.ReadLine().Split(' ')
                    .ForEach(val => _inputQueue.Enqueue(val));
            return _inputQueue.Dequeue();
        }

        public static implicit operator int(ConsoleInputExtension _)
        {
            if (_inputQueue.Count == 0)
                Console.ReadLine().Split(' ')
                    .ForEach(val => _inputQueue.Enqueue(val));
            return int.Parse(_inputQueue.Dequeue());
        }

        public static implicit operator long(ConsoleInputExtension _)
        {
            if (_inputQueue.Count == 0)
                Console.ReadLine().Split(' ')
                    .ForEach(val => _inputQueue.Enqueue(val));
            return long.Parse(_inputQueue.Dequeue());
        }

        public static implicit operator float(ConsoleInputExtension _)
        {
            if (_inputQueue.Count == 0)
                Console.ReadLine().Split(' ')
                    .ForEach(val => _inputQueue.Enqueue(val));
            return float.Parse(_inputQueue.Dequeue());
        }

        public static implicit operator double(ConsoleInputExtension _)
        {
            if (_inputQueue.Count == 0)
                Console.ReadLine().Split(' ')
                    .ForEach(val => _inputQueue.Enqueue(val));
            return double.Parse(_inputQueue.Dequeue());
        }
    }

    public static class ConsoleOutputExtension
    {
        private static StringBuilder _builder = new StringBuilder();

        public static void Cout(object o)
        {
            _builder.Append(o);
        }


        public static void CoutF(object o)
        {
            _builder.Append(o);
            Flush();
        }

        public static void Coutln(object o)
        {
            _builder.Append(o);
            _builder.Append('\n');
        }

        public static void Coutln()
        {
            _builder.Append('\n');
        }

        public static void CoutlnF(object o)
        {
            _builder.Append(o);
            _builder.Append('\n');
            Flush();
        }

        public static void Flush()
        {
            Console.Write(_builder.ToString());
            _builder.Clear();
        }
    }

    public static class MathExtension
    {
        //最小公倍数
        public static int LCM(int num1, int num2)
        {
            var gcd = GCD(num1, num2);
            return num1 * (num2 / gcd);
        }

        public static long LCM(long num1, long num2)
        {
            var gcd = GCD(num1, num2);
            return num1 * (num2 / gcd);
        }

        //最大公約数
        public static int GCD(int num1, int num2)
        {
            int a = Math.Max(num1, num2);
            int b = Math.Min(num1, num2);
            if (b == 0) return a;
            int mod;
            while ((mod = a % b) != 0)
            {
                a = b;
                b = mod;
            }

            return b;
        }

        public static long GCD(long num1, long num2)
        {
            long a = Math.Max(num1, num2);
            long b = Math.Min(num1, num2);
            if (b == 0) return a;
            long mod;
            while ((mod = a % b) != 0)
            {
                a = b;
                b = mod;
            }

            return b;
        }
    }

    public static class EnumerableExtension
    {
        public delegate void Function<in T>(T val);

        public static void ForEach<T>(this IEnumerable<T> enumerable, Function<T> function)
        {
            foreach (var x in enumerable)
            {
                function(x);
            }
        }
    }
}