pub mod binary_search {
    type T = usize;

    pub fn binary_search(mut ok: T, mut ng: T, f: impl Fn(T) -> bool) -> T {
        let cont = |ok: T, ng: T| {
            let l = ok.min(ng);
            let r = ok.max(ng);
            l + 1 < r
        };
        while cont(ok, ng) {
            let x = (ok + ng) / 2;
            if f(x) {
                ok = x;
            } else {
                ng = x;
            }
        }
        ok
    }
}

pub mod scanner {

    pub struct Scanner {
        buf: Vec<String>,
    }

    impl Scanner {
        pub fn new() -> Self {
            Self { buf: vec![] }
        }

        pub fn new_from(source: &str) -> Self {
            let source = String::from(source);
            let buf = Self::split(source);
            Self { buf }
        }

        pub fn next<T: std::str::FromStr>(&mut self) -> T {
            loop {
                if let Some(x) = self.buf.pop() {
                    return x.parse().ok().expect("");
                }
                let mut source = String::new();
                std::io::stdin().read_line(&mut source).expect("");
                self.buf = Self::split(source);
            }
        }

        fn split(source: String) -> Vec<String> {
            source
                .split_whitespace()
                .rev()
                .map(String::from)
                .collect::<Vec<_>>()
        }
    }
}

use crate::{binary_search::binary_search, scanner::Scanner};
use std::io::Write;
fn main() {
    let mut scanner = Scanner::new();
    let out = std::io::stdout();
    let mut out = std::io::BufWriter::new(out.lock());
    let t: usize = 1;
    for _ in 0..t {
        solve(&mut scanner, &mut out);
    }
}
fn solve(scanner: &mut Scanner, out: &mut std::io::BufWriter<std::io::StdoutLock>) {
    let h: usize = scanner.next();
    let w: usize = scanner.next();
    let s: Vec<Vec<char>> = (0..h)
        .map(|_| scanner.next::<String>().chars().map(|c| c).collect())
        .collect();
    let mut sum = vec![vec![0; w + 1]; h + 1];
    for i in 1..=h {
        for j in 1..=w {
            sum[i][j] = sum[i - 1][j] + sum[i][j - 1] - sum[i - 1][j - 1]
                + if s[i - 1][j - 1] == '#' { 1 } else { 0 };
        }
    }
    let ans = binary_search(1, h.min(w) + 1, |k| {
        let mut paint = vec![0isize; w];
        for i in 0..h {
            let mut now = vec![0; w + 1];
            if i + k <= h {
                let i2 = i + k;
                for j in 0..w - k + 1 {
                    let j2 = j + k;
                    let count = sum[i2][j2] - sum[i][j2] - sum[i2][j] + sum[i][j];
                    if count == k * k {
                        now[j] += 1;
                        now[j2] -= 1;
                    }
                }
            }
            for j in 0..w {
                if now[j] > 0 {
                    paint[j] = k as isize;
                }
                if s[i][j] == '#' && paint[j] <= 0 {
                    return false;
                }
                now[j + 1] += now[j];
                paint[j] -= 1;
            }
        }
        true
    });
    writeln!(out, "{}", ans).unwrap();
}