Problem:
Score : 400 points

Problem Statement

Takahashi has a maze, which is a grid of H \times W squares with H horizontal rows and W vertical columns.

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

From a road square, you can move to a horizontally or vertically adjacent road square.

You cannot move out of the maze, move to a wall square, or move diagonally.

Takahashi will choose a starting square and a goal square, which can be any road squares, and give the maze to Aoki.

Aoki will then travel from the starting square to the goal square, in the minimum number of moves required.

In this situation, find the maximum possible number of moves Aoki has to make.

Constraints

1 \leq H,W \leq 20

S_{ij} is . or #.

S contains at least two occurrences of ..

Any road square can be reached from any road square in zero or more moves.

Input

Input is given from Standard Input in the following format:

H W
S_{11}...S_{1W}
:
S_{H1}...S_{HW}

Output

Print the maximum possible number of moves Aoki has to make.

Sample Input 1

3 3
...
...
...

Sample Output 1

4

If Takahashi chooses the top-left square as the starting square and the bottom-right square as the goal square, Aoki has to make four moves.

Sample Input 2

3 5
...#.
.#.#.
.#...

Sample Output 2

10

If Takahashi chooses the bottom-left square as the starting square and the top-right square as the goal square, Aoki has to make ten moves.

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

using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Numerics;

namespace CompetitionPrograming2
{
    public sealed class Program : BaseProgram
    {
        public static void Main(string[] _) { using (new SetConsole()) { Solve(); } }
        static int h, w;
        static char[][] map;
        public static void Solve()
        {
            (h, w) = GetArray<int>();
            map = new char[h][];
            for (int i = 0; i < h; i++)
            {
                map[i] = GetString().ToArray();
            }

            var ans = 0;
            for (int i = 0; i < h; i++)
            {
                for (int j = 0; j < w; j++)
                {
                    if (map[i][j] == '#')
                    {
                        continue;
                    }

                    ans = Math.Max(ans, BFS(i, j));
                }
            }
            Write(ans);
        }

        private static int BFS(int i, int j)
        {
            var queue = new Queue<(int i, int j, int cost)>();
            queue.Enqueue((i, j, 0));
            var costs = new int[h][];
            for (int k = 0; k < h; k++)
            {
                costs[k] = Enumerable.Repeat(int.MaxValue, w).ToArray();
            }
            costs[i][j] = 0;

            while (queue.Any())
            {
                var (y, x, c) = queue.Dequeue();

                // 上
                if (y - 1 >= 0 && map[y - 1][x] == '.' && c + 1 < costs[y - 1][x])
                {
                    costs[y - 1][x] = c + 1;
                    queue.Enqueue((y - 1, x, c + 1));
                }
                // 下
                if (y + 1 < h && map[y + 1][x] == '.' && c + 1 < costs[y + 1][x])
                {
                    costs[y + 1][x] = c + 1;
                    queue.Enqueue((y + 1, x, c + 1));
                }
                // 右
                if (x + 1 < w && map[y][x + 1] == '.' && c + 1 < costs[y][x + 1])
                {
                    costs[y][x + 1] = c + 1;
                    queue.Enqueue((y, x + 1, c + 1));
                }
                // 左
                if (x - 1 >= 0 && map[y][x - 1] == '.' && c + 1 < costs[y][x - 1])
                {
                    costs[y][x - 1] = c + 1;
                    queue.Enqueue((y, x - 1, c + 1));
                }
            }

            var max = 0;
            for (int a = 0; a < h; a++)
            {
                for (int b = 0; b < w; b++)
                {
                    if (costs[a][b] == int.MaxValue)
                    {
                        continue;
                    }
                    max = Math.Max(max, costs[a][b]);
                }
            }
            return max;
        }
    }
    public abstract class BaseProgram
    {
        protected readonly static long divisor = 1000000007;
        protected static Action<object> Write => Console.WriteLine;
        protected static string GetString()
        {
            var str = Console.ReadLine();
            if (str is null) { throw new NullReferenceException("標準入力がnullです Solveメソッド内の解答コードが間違っています"); }
            return str;
        }
        protected static T GetNumber<T>() where T : IComparable, IComparable<T>, IConvertible, IEquatable<T>, IFormattable
        {
            var str = GetString();
            return Activator.CreateInstance<T>() switch
            {
                int _ => (T)(object)str.ToInt(),
                long _ => (T)(object)str.ToLong(),
                double _ => (T)(object)str.ToDouble(),
                decimal _ => (T)(object)str.ToDecimal(),
                BigInteger _ => (T)(object)str.ToBigInteger(),
                _ => throw new NotSupportedException(),
            };
        }
        protected static T[] GetArray<T>() where T : IComparable, IComparable<T>, IEquatable<T> => ToArray<T>(GetString());
        protected static List<T> GetList<T>() where T : IComparable, IComparable<T>, IEquatable<T> => ToList<T>(GetString());
        protected static void Swap<T>(ref T item1, ref T item2) => (item2, item1) = (item1, item2);
        protected static IEnumerable<char> AtoZ()
        {
            for (char i = 'a'; i <= 'z'; i++)
            {
                yield return i;
            }
        }
        private static T[] ToArray<T>(string str) where T : IComparable, IComparable<T>, IEquatable<T>
        {
            return ConvertEnumerator<T>(str).ToArray();
        }
        private static List<T> ToList<T>(string str) where T : IComparable, IComparable<T>, IEquatable<T>
        {
            return ConvertEnumerator<T>(str).ToList();
        }
        private static IEnumerable<T> ConvertEnumerator<T>(string str) where T : IComparable, IComparable<T>, IEquatable<T>
        {
            return Activator.CreateInstance<T>() switch
            {
                byte _ => str.Split().Select(byte.Parse).Cast<T>(),
                int _ => str.Split().Select(int.Parse).Cast<T>(),
                long _ => str.Split().Select(long.Parse).Cast<T>(),
                double _ => str.Split().Select(double.Parse).Cast<T>(),
                decimal _ => str.Split().Select(decimal.Parse).Cast<T>(),
                BigInteger _ => str.Split().Select(BigInteger.Parse).Cast<T>(),
                _ => throw new NotSupportedException()
            };
        }
        protected sealed class SetConsole : IDisposable
        {
            readonly StreamWriter sw = new StreamWriter(Console.OpenStandardOutput());
            public SetConsole()
            {
                sw.AutoFlush = false;
                Console.SetOut(sw);
            }
            public void Dispose()
            {
                Console.Out.Flush();
                sw.AutoFlush = true;
                Console.SetOut(sw);
            }
        }
    }
    public static class ExtentionsLibrary
    {
        public static long Choose(this int n, int a, long divisor = 1000000007)
        {
            long numerator = 1, denominator = 1;
            for (int i = 0; i < a; i++)
            {
                numerator = numerator % divisor * (n - i) % divisor;
                numerator %= divisor;
                denominator *= i + 1;
                denominator %= divisor;
            }
            return numerator / denominator;
        }
        /// <summary>
        /// 階乗を求める
        /// </summary>
        /// <param name="num"></param>
        /// <returns></returns>
        public static long Factorial(this long num)
        {
            var result = 1L;
            for (int i = 1; i <= num; i++)
            {
                result *= i;
            }
            return result;
        }
        private static T[] NextPermutation<T>(this IEnumerable<T> collection, IComparer<T> cmp) where T : IComparable
        {
            var array = collection.ToArray();
            int? exchangingNumIndex = null;
            for (int i = 1; i < array.Length; i++)
            {
                if (cmp.Compare(array[i - 1], array[i]) >= 0) { continue; }

                exchangingNumIndex = i - 1;
            }

            if (exchangingNumIndex is null) { return null; }

            for (int j = array.Length - 1; j >= 0; j--)
            {
                if (cmp.Compare(array[(int)exchangingNumIndex], array[j]) >= 0) { continue; }

                var tmp = array[j];
                array[j] = array[(int)exchangingNumIndex];
                array[(int)exchangingNumIndex] = tmp;
                array = array.Take((int)exchangingNumIndex + 1).Concat(array.Skip((int)exchangingNumIndex + 1).Reverse()).ToArray();
                break;
            }
            return array;
        }
        public static string[] NextPermutation(this IEnumerable<string> collection)
        {
            return NextPermutation(collection, StringComparer.Ordinal);
        }
        public static T[] NextPermutation<T>(this IEnumerable<T> collection) where T : IComparable
        {
            return NextPermutation(collection, Comparer<T>.Default);
        }
        public static IEnumerable<T[]> Permutations<T>(this IEnumerable<T> collection) where T : IComparable
        {
            var next = collection.NextPermutation();

            while (next != null)
            {
                yield return next;

                next = next.NextPermutation();
            }
        }
        public static long Gcd(this long a, long b)
        {
            if (a < b) { return Gcd(b, a); }
            long tmp;
            do
            {
                tmp = a % b;
                a = b;
                b = tmp;
            } while (tmp != 0);
            return a;
        }
        public static long Lcm(this long a, long b) => a * b / Gcd(a, b);
        /// <summary>
        /// 素因数分解を行う
        /// </summary>
        /// <param name="num"></param>
        /// <returns></returns>
        public static IEnumerable<(long num, int count)> Factorize(this long n)
        {
            yield return (1, 1);
            var rootN = Math.Sqrt(n);
            for (long i = 2; i <= rootN; i++)
            {
                if (n % i != 0) { continue; }

                var count = 1;
                n /= i;
                while (n % i == 0)
                {
                    n /= i;
                    count++;
                }
                yield return (i, count);
            }
            if (n != 1) { yield return (n, 1); }
        }
        public static int ToInt(this string str) => int.Parse(str);
        public static int ToInt(this char chr) => int.Parse(chr.ToString());
        public static long ToLong(this string str) => long.Parse(str);
        public static double ToDouble(this string str) => double.Parse(str);
        public static decimal ToDecimal(this string str) => decimal.Parse(str);
        public static BigInteger ToBigInteger(this string str) => BigInteger.Parse(str);
        public static DateTime ToDateTime(this string str) => DateTime.Parse(str);
        public static string StringJoin<T>(this IEnumerable<T> collection, string separator = "") => string.Join(separator, collection.Select(c => c.ToString()));
        public static string StringConcat(this IEnumerable<char> collection) => string.Concat(collection);
        public static int LowerBound<T>(this IReadOnlyList<T> a, T v) => LowerBound(a, v, Comparer<T>.Default);
        public static int LowerBound(this IReadOnlyList<string> str, string v) => LowerBound(str, v, StringComparer.Ordinal);
        public static int LowerBound<T>(this IReadOnlyList<T> a, T v, IComparer<T> cmp)
        {
            var l = 0;
            var r = a.Count - 1;
            while (l <= r)
            {
                var mid = l + (r - l) / 2;
                var result = cmp.Compare(a[mid], v);
                if (result == -1)
                {
                    l = mid + 1;
                }
                else
                {
                    r = mid - 1;
                }
            }
            return l;
        }
        public static int UpperBound(this IReadOnlyList<string> str, string v) => UpperBound(str, v, StringComparer.Ordinal);
        public static int UpperBound<T>(this IReadOnlyList<T> a, T v) => UpperBound(a, v, Comparer<T>.Default);
        public static int UpperBound<T>(this IReadOnlyList<T> a, T v, IComparer<T> cmp)
        {
            var l = 0;
            var r = a.Count - 1;
            while (l <= r)
            {
                var mid = l + (r - l) / 2;
                var res = cmp.Compare(a[mid], v);
                if (res <= 0) l = mid + 1;
                else r = mid - 1;
            }
            return l;
        }
        public static IOrderedEnumerable<TSource> OrderBy<TSource>(this IEnumerable<TSource> source, Func<TSource, string> keySelector)
        {
            return source.OrderBy(keySelector, StringComparer.Ordinal);
        }
        public static IOrderedEnumerable<TSource> OrderByDescending<TSource>(this IEnumerable<TSource> source, Func<TSource, string> keySelector)
        {
            return source.OrderByDescending(keySelector, StringComparer.Ordinal);
        }
        public static void Deconstruct<T>
        (
            this IList<T> self,
            out T first
        )
        {
            first = self[0];
        }

        public static void Deconstruct<T>
        (
            this IList<T> self,
            out T first,
            out T second
        )
        {
            first = self[0];
            second = self[1];
        }

        public static void Deconstruct<T>
        (
            this IList<T> self,
            out T first,
            out T second,
            out T third
        )
        {
            first = self[0];
            second = self[1];
            third = self[2];
        }

        public static void Deconstruct<T>
        (
            this IList<T> self,
            out T first,
            out T second,
            out T third,
            out T forth
        )
        {
            first = self[0];
            second = self[1];
            third = self[2];
            forth = self[3];
        }
        public static void Deconstruct<T>
        (
            this IList<T> self,
            out T first,
            out T second,
            out T third,
            out T forth,
            out T fifth
        )
        {
            first = self[0];
            second = self[1];
            third = self[2];
            forth = self[3];
            fifth = self[4];
        }
    }

    public struct MInt
    {
        private static readonly long MOD = (long)1e9 + 7;

        /// <summary>
        /// 0以上MOD未満の整数
        /// </summary>
        public long Value;

        public MInt(long val)
        {
            Value = val % MOD;
            if (Value < 0) Value += MOD;
        }

        private static MInt Ctor(long val) => new MInt() { Value = val };
        public static MInt operator +(MInt a, MInt b)
        {
            long res = a.Value + b.Value;
            if (res > MOD) res -= MOD;
            return Ctor(res);
        }
        public static MInt operator -(MInt a, MInt b)
        {
            long res = a.Value - b.Value;
            if (res < 0) res += MOD;
            return Ctor(res);
        }
        public static MInt operator *(MInt a, MInt b)
        {
            long res = a.Value * b.Value;
            if (res > MOD) res %= MOD;
            return Ctor(res);
        }
        public static MInt operator /(MInt a, MInt b) => a * Inv(b);
        public MInt Pow(long n)
        {
            if (n == 0) return new MInt(1);
            MInt b = new MInt(Value).Pow(n >> 1);
            b *= b;
            if ((n & 1) == 1) b *= Value;
            return b;
        }
        public static MInt Inv(MInt n)
        {
            long a = n.Value;
            long b = MOD;

            long x = 1;
            long u = 0;
            while (b != 0)
            {
                long k = a / b;
                long _x = u;
                u = x - k * u;
                x = _x;

                long _a = a;
                a = b;
                b = _a - (k * b);
            }

            return new MInt(x);
        }
        public override string ToString() => Value.ToString();
        public static implicit operator MInt(long a) => new MInt(a);
        public static explicit operator long(MInt a) => a.Value;
        public MInt Choose(long k)
        {
            MInt numerator = 1, denominator = 1;
            for (int i = 0; i < k; i++)
            {
                numerator *= new MInt(Value) - i;
                denominator *= i + 1;
            }
            return numerator / denominator;
        }
    }
    public enum Priority : byte
    {
        Smaller = 0,
        Larger = 1
    }
}