#![allow(unused_imports, unused_macros, dead_code)]

macro_rules! min {
    (.. $x:expr) => {{
        let mut it = $x.iter();
        it.next().map(|z| it.fold(z, |x, y| min!(x, y)))
    }};
    ($x:expr) => ($x);
    ($x:expr, $($ys:expr),*) => {{
        let t = min!($($ys),*);
        if $x < t { $x } else { t }
    }}
}
macro_rules! max {
    (.. $x:expr) => {{
        let mut it = $x.iter();
        it.next().map(|z| it.fold(z, |x, y| max!(x, y)))
    }};
    ($x:expr) => ($x);
    ($x:expr, $($ys:expr),*) => {{
        let t = max!($($ys),*);
        if $x > t { $x } else { t }
    }}
}


macro_rules! ewriteln {
    ($($args:expr),*) => { let _ = writeln!(&mut std::io::stderr(), $($args),*); };
}
macro_rules! trace {
    ($x:expr) => { ewriteln!(">>> {} = {:?}", stringify!($x), $x) };
    ($($xs:expr),*) => { trace!(($($xs),*)) }
}
macro_rules! put {
    (.. $x:expr) => {{
        let mut it = $x.iter();
        if let Some(x) = it.next() { print!("{}", x); }
        for x in it { print!(" {}", x); }
        println!("");
    }};
    ($x:expr) => { println!("{}", $x) };
    ($x:expr, $($xs:expr),*) => { print!("{} ", $x); put!($($xs),*) }
}

const M: i64 = 1_000_000_007;

use std::cmp::min;
fn wall(d: &mut Vec<Vec<i32>>) {
    let n = d.len();
    for k in 0..n {
        for i in 0..n {
            for j in 0..n {
                d[i][j] = min::<i32>(
                    d[i][j],
                    d[i][k] + d[k][j]
                );
            }
        }
    }
}

fn neigh(x: (usize, usize), y: (usize, usize)) -> bool {
    let dx = (x.0 as i64 - y.0 as i64).abs();
    let dy = (x.1 as i64 - y.1 as i64).abs();
    (dx == 1 && dy == 0) || (dx == 0 && dy == 1)
}

fn main() {
    let mut sc = Scanner::new();
    let h: usize = sc.cin();
    let w: usize = sc.cin();

    let mut vs = vec![];
    for i in 0..h {
        let line = sc.chars();
        for j in 0..w {
            if line[j] == '.' {
                vs.push((i, j));
            }
        }
    }
    let n = vs.len();
    // trace!(n);

    let inf = 400 * 400 + 100;
    let mut d = vec![vec![inf; n]; n];
    for i in 0..n {
        d[i][i] = 0;
    }
    for i in 0..n {
        for j in 0..i {
            if neigh(vs[j], vs[i]) {
                d[i][j] = 1;
                d[j][i] = 1;
            }
        }
    }
    wall(&mut d);
    let mut ans = 0;
    for i in 0..n {
        for j in 0..n {
            ans = max!(ans, d[i][j]);
        }
    }
    put!(ans);
}

use std::io::{self, Write};
use std::str::FromStr;
use std::collections::VecDeque;

struct Scanner { stdin: io::Stdin, buffer: VecDeque<String>, }
impl Scanner {
    fn new() -> Self { Scanner { stdin: io::stdin(), buffer: VecDeque::new() } }
    fn cin<T: FromStr>(&mut self) -> T {
        while self.buffer.len() == 0 {
            let mut line = String::new();
            let _ = self.stdin.read_line(&mut line);
            for w in line.split_whitespace() {
                self.buffer.push_back(String::from(w));
            }
        }
        self.buffer.pop_front().unwrap().parse::<T>().ok().unwrap()
    }
    fn chars(&mut self) -> Vec<char> { self.cin::<String>().chars().collect() }
    fn vec<T: FromStr>(&mut self, n: usize) -> Vec<T> { (0..n).map(|_| self.cin()).collect() }
}


Measured memory_bytes: 4352