#[macro_use]
mod input_mcr {
    // ref: tanakh <https://qiita.com/tanakh/items/0ba42c7ca36cd29d0ac8>
    // diff: using Parser
    #[macro_export(local_inner_macros)]
    macro_rules! input {
    (source = $s:expr, $($r:tt)*) => {
        let mut parser = Parser::from_str($s);
        input_inner!{parser, $($r)*}
    };
    (parser = $parser:ident, $($r:tt)*) => {
        input_inner!{$parser, $($r)*}
    };
    (new_stdin_parser = $parser:ident, $($r:tt)*) => {
        let stdin = std::io::stdin();
        let reader = std::io::BufReader::new(stdin.lock());
        let mut $parser = Parser::new(reader);
        input_inner!{$parser, $($r)*}
    };
    ($($r:tt)*) => {
        input!{new_stdin_parser = parser, $($r)*}
    };
}

    #[macro_export(local_inner_macros)]
    macro_rules! input_inner {
    ($parser:ident) => {};
    ($parser:ident, ) => {};
    ($parser:ident, $var:ident : $t:tt $($r:tt)*) => {
        let $var = read_value!($parser, $t);
        input_inner!{$parser $($r)*}
    };
}

    #[macro_export(local_inner_macros)]
    macro_rules! read_value {
    ($parser:ident, ( $($t:tt),* )) => {
        ( $(read_value!($parser, $t)),* )
    };
    ($parser:ident, [ $t:tt ; $len:expr ]) => {
        (0..$len).map(|_| read_value!($parser, $t)).collect::<Vec<_>>()
    };
    ($parser:ident, chars) => {
        read_value!($parser, String).chars().collect::<Vec<char>>()
    };
    ($parser:ident, char_) => {
        read_value!($parser, String).chars().collect::<Vec<char>>()[0]
    };
    ($parser:ident, usize1) => {
        read_value!($parser, usize) - 1
    };
    ($parser:ident, i64_) => {
        $parser.fast_i64()
    };
    ($parser:ident, usize_) => {
        $parser.fast_i64() as usize
    };
    ($parser:ident, usize1_) => {
        ($parser.fast_i64() - 1) as usize
    };
    ($parser:ident, $t:ty) => {
        $parser.next::<$t>().expect("Parse error")
    };
}

    use std::io;
    use std::io::BufRead;
    use std::str;

    // ref: tatsuya6502 <https://qiita.com/tatsuya6502/items/cd448486f7ef7b5b8c7e>
    // ref: wariuni <https://qiita.com/tatsuya6502/items/cd448486f7ef7b5b8c7e#comment-7040a5ae96305e884eb9>
    // diff: using std::io::BufRead::fill_buf()
    pub struct Parser<R> {
        pub reader: R,
        buf: Vec<u8>,
        pos: usize,
    }

    impl Parser<io::Empty> {
        pub fn from_str(s: &str) -> Parser<io::Empty> {
            Parser {
                reader: io::empty(),
                buf: s.as_bytes().to_vec(),
                pos: 0,
            }
        }
    }

    impl<R: BufRead> Parser<R> {
        pub fn new(reader: R) -> Parser<R> {
            Parser {
                reader: reader,
                buf: vec![],
                pos: 0,
            }
        }
        pub fn update_buf(&mut self) {
            self.buf.clear();
            self.pos = 0;
            loop {
                let (len, complete) = {
                    let buf2 = self.reader.fill_buf().unwrap();
                    self.buf.extend_from_slice(buf2);
                    let len = buf2.len();
                    (len, buf2[len - 1] <= 0x20)
                };
                self.reader.consume(len);
                if complete {
                    break;
                }
            }
        }
        pub fn next<T: str::FromStr>(&mut self) -> Result<T, T::Err> {
            loop {
                let mut begin = self.pos;
                while begin < self.buf.len() && (self.buf[begin] <= 0x20) {
                    begin += 1;
                }
                let mut end = begin;
                while end < self.buf.len() && (self.buf[end] > 0x20) {
                    end += 1;
                }
                if begin != self.buf.len() {
                    self.pos = end;
                    return unsafe { str::from_utf8_unchecked(&self.buf[begin..end]) }.parse::<T>();
                } else {
                    self.update_buf();
                }
            }
        }
        pub fn fast_i64(&mut self) -> i64 {
            loop {
                let mut begin = self.pos;
                while begin < self.buf.len() && (self.buf[begin] <= 0x20) {
                    begin += 1;
                }
                if begin == self.buf.len() {
                    self.update_buf();
                    continue;
                }
                let mut res = 0;
                let (is_positive, mut end) = match self.buf[begin] {
                    b'+' => (true, begin + 1),
                    b'-' => (false, begin + 1),
                    _ => (true, begin),
                };
                while end < self.buf.len() && (self.buf[end] > 0x20) {
                    res = res * 10 + (self.buf[end] as i64 - '0' as i64);
                    end += 1;
                }
                if begin != self.buf.len() {
                    self.pos = end;
                    return if is_positive { res } else { -res };
                } else {
                    self.update_buf();
                }
            }
        }
    }
}

use input_mcr::*;
use std::cmp::*;

use std::collections::VecDeque;

pub fn gen_d4(c: usize, r: usize, w: usize, h: usize) -> Vec<(usize, usize)> {
    let mut res = vec![];
    if r > 0 {
        res.push((c, r - 1));
    }
    if c > 0 {
        res.push((c - 1, r));
    }
    if r + 1 < h {
        res.push((c, r + 1));
    }
    if c + 1 < w {
        res.push((c + 1, r));
    }
    res
}

fn gen_dist_table(
    field: &Vec<Vec<char>>,
    px: usize,
    py: usize,
    w: usize,
    h: usize,
) -> Vec<Vec<i64>> {
    let mut q = VecDeque::new();
    let inf = 1i64 << 50;
    let mut dist = vec![vec![inf; w]; h];
    q.push_back((px, py));
    dist[py][px] = 0;
    while let Some((x, y)) = q.pop_front() {
        for &(nx, ny) in &gen_d4(x, y, w, h) {
            if field[ny][nx] == '.' && dist[ny][nx] > dist[y][x] + 1 {
                dist[ny][nx] = dist[y][x] + 1;
                q.push_back((nx, ny));
            }
        }
    }
    dist
}

fn main() {
    input! {
        h: usize,
        w: usize,
        field: [chars; h],
    }
    let mut res = 0;
    for y in 0..h {
        for x in 0..w {
            if field[y][x] == '.' {
                let dists = gen_dist_table(&field, x, y, w, h);
                for ny in 0..h {
                    for nx in 0..w {
                        if field[ny][nx] == '.' {
                            res = max(res, dists[ny][nx]);
                        }
                    }
                }
            }
        }
    }
    println!("{}", res);
    return;
    /*
    let (mut px, mut py) = (0,0);
    for y in 0..h {
        for x in 0..w {
            if field[y][x] == '.' {
                px = x;
                py = y;
            }
        }
    }
    let mut q = VecDeque::new();
    let inf = 1i64 << 50;
    let mut dist = vec![vec![inf; w]; h];
    q.push_back((px,py));
    dist[py][px] = 0;
    while let Some((x,y)) = q.pop_front() {
        for &(nx,ny) in &gen_d4(x,y,w,h) {
            if field[ny][nx] == '.' && dist[ny][nx] > dist[y][x] + 1 {
                dist[ny][nx] = dist[y][x] + 1;
                q.push_back((nx,ny));
            }
        }
    }
    for y in 0..h {
        for x in 0..w {
            if field[y][x] == '.' && dist[py][px] < dist[y][x] {
                px = x;
                py = y;
            }
        }
    }
    // eprintln!("px = {}, py = {}", px, py);
    let mut q = VecDeque::new();
    let inf = 1i64 << 50;
    let mut dist = vec![vec![inf; w]; h];
    q.push_back((px,py));
    dist[py][px] = 0;
    while let Some((x,y)) = q.pop_front() {
        for &(nx,ny) in &gen_d4(x,y,w,h) {
            if field[ny][nx] == '.' && dist[ny][nx] > dist[y][x] + 1 {
                dist[ny][nx] = dist[y][x] + 1;
                q.push_back((nx,ny));
            }
        }
    }
    let mut res = 0;
    for y in 0..h {
        for x in 0..w {
            if field[y][x] == '.' {
                if res < dist[y][x] {
                    res = dist[y][x];
                }
            }
        }
    }
    println!("{}", res);
    */
}



Measured memory_bytes: 4352