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 Rust (1.15.1) (judge verdict: Accepted)

//  ____            _                  _                       _       _
// |  _ \ _   _ ___| |_    ___ _ __   | |_ ___ _ __ ___  _ __ | | __ _| |_ ___
// | |_) | | | / __| __|  / __| '_ \  | __/ _ \ '_ ` _ \| '_ \| |/ _` | __/ _ \
// |  _ <| |_| \__ \ |_  | (__| |_) | | ||  __/ | | | | | |_) | | (_| | ||  __/
// |_| \_\\__,_|___/\__|  \___| .__/___\__\___|_| |_| |_| .__/|_|\__,_|\__\___|
//                            |_| |_____|               |_|

//https://github.com/manta1130/Competitive_Programming_Template_Rust

#[macro_use]
mod input {

    use std;
    use std::io;

    const SPLIT_DELIMITER: char = ' ';

    #[macro_export]
    #[allow(unused_macros)]
    macro_rules! input {
    ( $($x:expr ),*) => {
        {
            let temp_str = input_line_str();
            let mut split_result_iter = temp_str.split_whitespace();
                $(
                let buf_split_result = split_result_iter.next();
                let buf_split_result = buf_split_result.unwrap();
                    ($x) = buf_split_result.parse().unwrap();
                )*
        }
    };
}

    #[allow(dead_code)]
    pub fn input_line_str() -> String {
        let mut s = String::new();
        io::stdin().read_line(&mut s).unwrap();
        s.trim().to_string()
    }

    #[allow(dead_code)]
    pub fn p<T>(t: T)
    where
        T: std::fmt::Display,
    {
        println!("{}", t);
    }

    #[allow(dead_code)]
    pub fn input_vector2d<T>(line: usize) -> Vec<Vec<T>>
    where
        T: std::str::FromStr,
    {
        let mut v: Vec<Vec<T>> = Vec::new();

        for _ in 0..line {
            let vec_line = input_vector();
            v.push(vec_line);
        }
        v
    }

    #[allow(dead_code)]
    pub fn input_vector<T>() -> Vec<T>
    where
        T: std::str::FromStr,
    {
        let mut v: Vec<T> = Vec::new();

        let s = input_line_str();
        let split_result = s.split(SPLIT_DELIMITER);
        for z in split_result {
            let buf = match z.parse() {
                Ok(r) => r,
                Err(_) => panic!("Parse Error"),
            };
            v.push(buf);
        }
        v
    }

    #[allow(dead_code)]
    pub fn input_vector_row<T>(n: usize) -> Vec<T>
    where
        T: std::str::FromStr,
    {
        let mut v = Vec::with_capacity(n);
        for _ in 0..n {
            let buf = match input_line_str().parse() {
                Ok(r) => r,
                Err(_) => panic!("Parse Error"),
            };
            v.push(buf);
        }
        v
    }

    pub trait ToCharVec {
        fn to_charvec(&self) -> Vec<char>;
    }

    impl ToCharVec for String {
        fn to_charvec(&self) -> Vec<char> {
            self.to_string().chars().collect::<Vec<_>>()
        }
    }
}

use input::*;
use std::collections::VecDeque;

const INF: usize = 999999999;

fn bfs(x: usize, y: usize, v: &Vec<Vec<char>>) -> usize {
    let mut m = vec![vec![INF; v[0].len()]; v.len()];
    let xl = v[0].len();
    let yl = v.len();
    m[y][x] = 0;

    let mut q = VecDeque::new();
    q.push_front((x, y, 0));

    while !q.is_empty() {
        let (xx, yy, c) = q.pop_back().unwrap();
        if m[yy][xx] < c {
            continue;
        }
        m[yy][xx] = c;
        if yy + 1 < yl && m[yy + 1][xx] > c + 1 && v[yy + 1][xx] == '.' {
            m[yy + 1][xx] = c + 1;
            q.push_front((xx, yy + 1, c + 1));
        }
        if xx + 1 < xl && m[yy][xx + 1] > c + 1 && v[yy][xx + 1] == '.' {
            m[yy][xx + 1] = c + 1;
            q.push_front((xx + 1, yy, c + 1));
        }
        if yy != 0 && m[yy - 1][xx] > c + 1 && v[yy - 1][xx] == '.' {
            m[yy - 1][xx] = c + 1;
            q.push_front((xx, yy - 1, c + 1));
        }
        if xx != 0 && m[yy][xx - 1] > c + 1 && v[yy][xx - 1] == '.' {
            m[yy][xx - 1] = c + 1;
            q.push_front((xx - 1, yy, c + 1));
        }
    }

    let mut res = 0;
    for i in m {
        for j in i {
            if j != INF {
                res = std::cmp::max(res, j);
            }
        }
    }

    res
}

fn main() {
    let (h, w): (usize, usize);
    input!(h, w);

    let mut v = vec![];
    for _ in 0..h {
        let s = input_line_str().to_charvec();
        v.push(s);
    }

    let mut ans = 0;

    for i in 0..h {
        for j in 0..w {
            if v[i][j] == '#' {
                continue;
            }
            ans = std::cmp::max(ans, bfs(j, i, &v));
        }
    }

    p(ans);
}