#[allow(unused_macros)]
macro_rules! input {
    (source = $s:expr, $($r:tt)*) => {
        let mut iter = $s.split_whitespace();
        let mut next = || { iter.next().unwrap() };
        input_inner!{next, $($r)*}
    };
    ($($r:tt)*) => {
        let stdin = std::io::stdin();
        let mut bytes = std::io::Read::bytes(std::io::BufReader::new(stdin.lock()));
        let mut next = move || -> String{
            bytes
                .by_ref()
                .map(|r|r.unwrap() as char)
                .skip_while(|c|c.is_whitespace())
                .take_while(|c|!c.is_whitespace())
                .collect()
        };
        input_inner!{next, $($r)*}
    };
}

#[allow(unused_macros)]
macro_rules! input_inner {
    ($next:expr) => {};
    ($next:expr, ) => {};

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

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

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

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

    ($next:expr, chars) => {
        read_value!($next, String).chars().collect::<Vec<char>>()
    };

    ($next:expr, bytes) => {
        read_value!($next, String).into_bytes()
    };

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

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

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

    let mut dist = vec![vec![1000_000_000; h*w]; h*w];

    for y in 0..h {
        for x in 0..w {
            if s[y][x] == '#' {
                continue;
            }
            for &(dx, dy) in [(0, 1), (0, -1), (1, 0), (-1, 0)].iter() {
                let cx = x as i64 + dx;
                let cy = y as i64 + dy;

                if !(cx >= 0 && cx < w as i64 && cy >= 0 && cy < h as i64) {
                    continue;
                }

                if s[cy as usize][cx as usize] == '.' {
                    let f = (cy * w as i64 + cx) as usize;
                    let t = y * w + x;
                    dist[f][t] = 1;
                    dist[t][f] = 1;
                }
            }
        }
    }

    for k in 0..h*w {
        for i in 0..h*w {
            for j in 0..h*w {
                dist[i][j] = std::cmp::min(dist[i][j], dist[i][k] + dist[k][j]);
            }
        }
    }

    let mut ans = 0;
    for i in 0..h*w {
        for j in 0..h*w {
            if dist[i][j] != 1000_000_000 {
                ans = std::cmp::max(ans, dist[i][j]);
            }
        }
    }

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