use std::collections::VecDeque;

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)*}
    };
}

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<_>>()
    };

    ($iter:expr, chars) => {
        read_value!($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")
    };
}

#[derive(Debug)]
struct T {
    x: i32,
    y: i32,
    dist: usize,
}

const dxs: [i32; 4] = [0, 0, 1, -1];
const dys: [i32; 4] = [1, -1, 0, 0];

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

    let mut ans = 0;
    for y in 0..(h as i32) {
        for x in 0..(w as i32) {
            let mut q = VecDeque::new();
            let mut used = vec![vec![0; w]; h];
            {
                let t = T {
                    x: x,
                    y: y,
                    dist: 0,
                };
                q.push_back(t);
            }
            let mut max = 0;

            while !q.is_empty() {
                let t = q.pop_front().unwrap();
                for i in 0..4 {
                    let nx = t.x + dxs[i];
                    let ny = t.y + dys[i];
                    if nx < 0 || nx >= (w as i32) {
                        continue;
                    }
                    if ny < 0 || ny >= (h as i32) {
                        continue;
                    }
                    if s[ny as usize][nx as usize] == '#' {
                        continue;
                    }
                    if used[ny as usize][nx as usize] == 1 {
                        continue;
                    }
                    used[ny as usize][nx as usize] = 1;

                    let ndist = t.dist + 1;
                    let nt = T {
                        x: nx,
                        y: ny,
                        dist: ndist,
                    };
                    q.push_back(nt);
                    if max < ndist {
                        max = ndist;
                    }
                }
            }
            if ans < max {
                ans = max;
            }
        }
    }
    println!("{}", ans);
}
