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

//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);
}


Measured memory_bytes: 4476