#[allow(unused_imports)]
use std::cmp::{Ordering};

#[allow(unused_imports)]
use std::collections::{BTreeMap, BTreeSet, BinaryHeap, HashMap, HashSet, VecDeque};

#[allow(unused_imports)]
use std::io::{stdin, stdout, BufWriter, Write};

#[allow(unused_imports)]
use std::iter::FromIterator;

#[allow(unused_macros)]
macro_rules! min {
    ($x: expr) => ($x);
    ($x: expr, $($z: expr),+) => {{
        std::cmp::min($x, min!($($z),*))
    }}
}

#[allow(unused_macros)]
macro_rules! max {
    ($x: expr) => ($x);
    ($x: expr, $($z: expr),+) => {{
        std::cmp::max($x, max!($($z),*))
    }}
}

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

#[allow(unused_macros)]
macro_rules! debug {
    ($($a:expr),*) => {
        #[cfg(debug_assertions)]
        writeln!(&mut std::io::stderr(), concat!("[DEBUG] ", $(stringify!($a), "={:?} "),*), $($a),*).unwrap();
    }
}

const BIG_STACK_SIZE: bool = true;

#[allow(dead_code)]
fn main() {
    use std::thread;
    if BIG_STACK_SIZE {
        thread::Builder::new()
            .stack_size(32 * 1024 * 1024)
            .name("solve".into())
            .spawn(solve)
            .unwrap()
            .join()
            .unwrap();
    } else {
        solve();
    }
}

fn solve() {
    let (h,w) = get!(usize, usize);
    let ss = get!(String; h);

    let mut s = Vec::new();
    for e in ss {
        s.push(e.chars().collect::<Vec<_>>());
    }

    let num = h*w;
    let mut nodes = vec![vec![false; num];num];
    const dx: [usize; 2] = [0,1];
    const dy: [usize; 2] = [1,0];

    for i in 0..h {
        for j in 0..w {
            if s[i][j] == '#' {
                continue;
            }
            let index = i*w+j;
            for k in 0..2 {
                let nx = j + dx[k];
                let ny = i + dy[k];
                if nx > w-1 || ny > h-1 {
                    continue;
                }
                if s[ny][nx] == '#' {
                    continue;
                }
                let nindex = ny*w+nx;
                nodes[index][nindex] = true;
                nodes[nindex][index] = true;
            }
        }
    }

    const INF: usize = 1_000_000_000_000_000;
    let mut dist = vec![vec![INF; num]; num];
    for i in 0..num {
        for j in 0..num {
            if i == j {
                dist[i][j] = 0;
            }
            if nodes[i][j] {
                dist[i][j] = 1;
            }
        }
    }

    for k in 0..num {
        for i in 0..num {
            for j in 0..num {
                dist[i][j] = min!(dist[i][k]+dist[k][j], dist[i][j]);
            }
        }
    }
    let mut mx = 0;
    for i in 0..num {
        for j in 0..num {
            if dist[i][j] != INF {
                mx = max!(mx, dist[i][j]);
            }
        }
    }
    println!("{}", mx);
}


Measured memory_bytes: 8572