#ifndef LOCAL
#define FAST_IO
#endif

// ============
#include <algorithm>
#include <array>
#include <bitset>
#include <cassert>
#include <cmath>
#include <iomanip>
#include <iostream>
#include <list>
#include <map>
#include <numeric>
#include <queue>
#include <random>
#include <set>
#include <stack>
#include <string>
#include <tuple>
#include <unordered_map>
#include <unordered_set>
#include <utility>
#include <vector>

#define OVERRIDE(a, b, c, d, ...) d
#define REP2(i, n) for (i32 i = 0; i < (i32)(n); ++i)
#define REP3(i, m, n) for (i32 i = (i32)(m); i < (i32)(n); ++i)
#define REP(...) OVERRIDE(__VA_ARGS__, REP3, REP2)(__VA_ARGS__)
#define PER(i, n) for (i32 i = (i32)(n) - 1; i >= 0; --i)
#define ALL(x) begin(x), end(x)

using namespace std;

using u32 = unsigned int;
using u64 = unsigned long long;
using i32 = signed int;
using i64 = signed long long;
using f64 = double;
using f80 = long double;

template <typename T>
using Vec = vector<T>;

template <typename T>
bool chmin(T &x, const T &y) {
    if (x > y) {
        x = y;
        return true;
    }
    return false;
}
template <typename T>
bool chmax(T &x, const T &y) {
    if (x < y) {
        x = y;
        return true;
    }
    return false;
}

#ifdef INT128

using u128 = __uint128_t;
using i128 = __int128_t;

istream &operator>>(istream &is, i128 &x) {
    i64 v;
    is >> v;
    x = v;
    return is;
}
ostream &operator<<(ostream &os, i128 x) {
    os << (i64)x;
    return os;
}
istream &operator>>(istream &is, u128 &x) {
    u64 v;
    is >> v;
    x = v;
    return is;
}
ostream &operator<<(ostream &os, u128 x) {
    os << (u64)x;
    return os;
}

#endif

[[maybe_unused]] constexpr i32 INF = 1000000100;
[[maybe_unused]] constexpr i64 INF64 = 3000000000000000100;
struct SetUpIO {
    SetUpIO() {
#ifdef FAST_IO
        ios::sync_with_stdio(false);
        cin.tie(nullptr);
#endif
        cout << fixed << setprecision(15);
    }
} set_up_io;
// ============

#ifdef DEBUGF
#else
#define DBG(x) (void)0
#endif

int main() {
    i32 h, w;
    cin >> h >> w;
    Vec<string> s(h);
    REP(i, h) {
        cin >> s[i];
    }
    Vec<Vec<i32>> sum(h + 1, Vec<i32>(w + 1, 0));
    REP(i, h) REP(j, w) {
        sum[i + 1][j + 1] = sum[i][j + 1] + sum[i + 1][j] - sum[i][j] + (s[i][j] == '#');
    }
    i32 ok = 1, ng = max(h, w) + 1;
    const auto judge = [&](i32 x) -> bool {
        Vec<Vec<i32>> add(h + 1, Vec<i32>(w + 1, 0));
        REP(i, h - x + 1) REP(j, w - x + 1) {
            i32 s = sum[i + x][j + x] - sum[i][j + x] - sum[i + x][j] + sum[i][j];
            if (s == x * x) {
                add[i][j] += 1;
                add[i + x][j] -= 1;
                add[i][j + x] -= 1;
                add[i + x][j + x] += 1;
            }
        }
        REP(i, h) REP(j, w + 1) {
            add[i + 1][j] += add[i][j];
        }
        REP(i, h + 1) REP(j, w) {
            add[i][j + 1] += add[i][j];
        }
        REP(i, h) REP(j, w) {
            char c = add[i][j] ? '#' : '.';
            if (c != s[i][j]) {
                return false;
            }
        }
        return true;
    };
    while (ng - ok > 1) {
        i32 mid = (ok + ng) / 2;
        if (judge(mid)) {
            ok = mid;
        } else {
            ng = mid;
        }
    }
    cout << ok << '\n';
}