#![allow(unused_parens)]
#![allow(unused_imports)]
#![allow(non_upper_case_globals)]

#[allow(unused_macros)]
macro_rules! invec {
    ( $ t : ty ) => {{
        let mut s = String::new();
        match std::io::stdin().read_line(&mut s) {
            Ok(0) => Vec::<$t>::new(),
            Ok(n) => s
                .trim()
                .split_whitespace()
                .map(|s| s.parse::<$t>().unwrap())
                .collect::<Vec<$t>>(),
            Err(_) => Vec::<$t>::new(),
        }
    }};
}

#[allow(unused_macros)]
macro_rules! get {
      ($t:ty) => {
          {
              let mut line: String = String::new();
              std::io::stdin().read_line(&mut line).unwrap();
              line.trim().parse::<$t>().unwrap()
          }
      };
      ($($t:ty),*) => {
          {
              let mut line: String = String::new();
              std::io::stdin().read_line(&mut line).unwrap();
              let mut iter = line.split_whitespace();
              (
                  $(iter.next().unwrap().parse::<$t>().unwrap(),)*
              )
          }
      };
      ($t:ty; $n:expr) => {
          (0..$n).map(|_|
              get!($t)
          ).collect::<Vec<_>>()
      };
      ($($t:ty),*; $n:expr) => {
          (0..$n).map(|_|
              get!($($t),*)
          ).collect::<Vec<_>>()
      };
      ($t:ty ;;) => {
          {
              let mut line: String = String::new();
              std::io::stdin().read_line(&mut line).unwrap();
              line.split_whitespace()
                  .map(|t| t.parse::<$t>().unwrap())
                  .collect::<Vec<_>>()
          }
      };
      ($t:ty ;; $n:expr) => {
          (0..$n).map(|_| get!($t ;;)).collect::<Vec<_>>()
      };
}

#[allow(unused_macros)]
macro_rules! input {
    (source = $s:expr, $($r:tt)*) => {
        let mut iter = $s.split_whitespace();
        input_inner!{iter, $($r)*}
    };
    ($($r:tt)*) => {
        let mut s = {
            use std::io::Read;
            let mut s = String::new();
            std::io::stdin().read_to_string(&mut s).unwrap();
            s
        };
        let mut iter = s.split_whitespace();
        input_inner!{iter, $($r)*}
    };
}

macro_rules! input_inner {
    ($iter:expr) => {};
    ($iter:expr, ) => {};

    ($iter:expr, $var:ident : $t:tt $($r:tt)*) => {
        let $var = read_value!($iter, $t);
        input_inner!{$iter $($r)*}
    };
}

#[allow(unused_macros)]
macro_rules! read_value {
    ($iter:expr, ( $($t:tt),* )) => {
        ( $(read_value!($iter, $t)),* )
    };

    ($iter:expr, [ $t:tt ; $len:expr ]) => {
        (0..$len).map(|_| read_value!($iter, $t)).collect::<Vec<_>>()
    };

    ($next:expr, [$t:tt]) => {
        {
            let len = read_value!($next, usize);
            (0..len).map(|_| read_value!($next, $t)).collect::<Vec<_>>()
        }
    };
    ($iter:expr, chars) => {
        read_value!($iter, String).chars().collect::<Vec<char>>()
    };

    ($iter:expr, usize1) => {
        read_value!($iter, usize) - 1
    };

    ($iter:expr, $t:ty) => {
        $iter.next().unwrap().parse::<$t>().expect("Parse error")
    };
}

#[cfg(debug_assertions)]
macro_rules! mydbg {
    //($arg:expr) => (dbg!($arg))
    //($arg:expr) => (println!("{:?}",$arg));
      ($($a:expr),*) => {
          eprintln!(concat!($(stringify!($a), " = {:?}, "),*), $($a),*);
      }
}
#[cfg(not(debug_assertions))]
macro_rules! mydbg {
    ($($arg:expr),*) => {};
}

use std::cmp;
use std::collections::*;
use std::ops::{Add, Div, Mul, Sub};

#[allow(dead_code)]
static INF_I64: i64 = 9223372036854775807;
#[allow(dead_code)]
static INF_I32: i32 = 2147483647;
#[allow(dead_code)]
static INF_USIZE: usize = 18446744073709551615;

fn main() {
    input! {
       h: usize,
       w:usize,
       s:[chars;h],
    }

    let mut ans = 0;

    for i in 0..h {
        for j in 0..w {
            if s[i][j] == '#' {
                continue;
            }
            let mut map = vec![vec![-1; w]; h];

            let mut kouho = VecDeque::new();
            let mut start = (j, i);
            map[i][j] = 0;

            kouho.push_back(start);
            while !kouho.is_empty() {
                let (x, y) = kouho.pop_front().unwrap();
                if x > 0 && s[y][x - 1] != '#' && map[y][x - 1] == -1 {
                    kouho.push_back((x - 1, y));
                    map[y][x - 1] = map[y][x] + 1;
                }
                if y > 0 && s[y - 1][x] != '#' && map[y - 1][x] == -1 {
                    kouho.push_back((x, y - 1));
                    map[y - 1][x] = map[y][x] + 1;
                }
                if x < w - 1 && s[y][x + 1] != '#' && map[y][x + 1] == -1 {
                    kouho.push_back((x + 1, y));
                    map[y][x + 1] = map[y][x] + 1;
                }
                if y < h - 1 && s[y + 1][x] != '#' && map[y + 1][x] == -1 {
                    kouho.push_back((x, y + 1));
                    map[y + 1][x] = map[y][x] + 1;
                }
            }

            //mydbg!(map);
            let mut m = -1;
            for i in 0..h {
                for j in 0..w {
                    if map[i][j] > m {
                        m = map[i][j];
                        //start = (j, i);
                    }
                }
            }
            ans = std::cmp::max(ans, m);
            mydbg!(map);
        }
    }

    println!("{}", ans);
}


Measured memory_bytes: 4352