// #pragma GCC target("avx")
// #pragma GCC optimize("O3")
// #pragma GCC optimize("unroll-loops")

#include <bits/stdc++.h>
using namespace std;

#define rep(i,n) for(int i = 0; i < (int)n; i++)
#define FOR(n) for(int i = 0; i < (int)n; i++)
#define repi(i,a,b) for(int i = (int)a; i < (int)b; i++)
#define all(x) x.begin(),x.end()
//#define mp make_pair
#define vi vector<int>
#define vvi vector<vi>
#define vvvi vector<vvi>
#define vvvvi vector<vvvi>
#define pii pair<int,int>
#define vpii vector<pair<int,int>>

template<typename T>
void chmax(T &a, const T &b) {a = (a > b? a : b);}
template<typename T>
void chmin(T &a, const T &b) {a = (a < b? a : b);}

using ll = long long;
using ld = long double;
using ull = unsigned long long;

const ll INF = numeric_limits<long long>::max() / 2;
const ld pi = 3.1415926535897932384626433832795028;
const ll mod = 998244353;
int dx[] = {1, 0, -1, 0, -1, -1, 1, 1};
int dy[] = {0, 1, 0, -1, -1, 1, -1, 1};

#define int long long

void solve() {
    int h, w;
    cin >> h >> w;
    vector<string> s(h);
    FOR(h) cin >> s[i];

    vvi tot(h+1, vi(w+1, 0));
    rep(i, h) rep(j, w) {
        tot[i+1][j+1] = tot[i][j+1] + tot[i+1][j] - tot[i][j] + (s[i][j] == '#');
    }

    int l = 0, r = min(h, w)+1;
    while(r-l > 1) {
        int k = (l+r)/2;

        vvi cur_tot(h+1, vi(w+1, 0));

        rep(i, h-k+1) {
            rep(j, w-k+1) {
                int sq_sum = tot[i+k][j+k] - tot[i][j+k] - tot[i+k][j] + tot[i][j];
                if(sq_sum == k*k) {
                    cur_tot[i][j]++;
                    cur_tot[i+k][j]--;
                    cur_tot[i][j+k]--;
                    cur_tot[i+k][j+k]++;
                }
            }
        }

        rep(i, h+1) rep(j, w+1) {
            if(i > 0) cur_tot[i][j] += cur_tot[i-1][j];
            if(j > 0) cur_tot[i][j] += cur_tot[i][j-1];
            if(i > 0 && j > 0) cur_tot[i][j] -= cur_tot[i-1][j-1];
        }

        vector<string> t(h, string(w, '@'));
        rep(i, h) rep(j, w) t[i][j] = (cur_tot[i][j] > 0? '#' : '.');
        
        if(s == t) l = k;
        else r = k;
    }

    cout << l << endl;
}

signed main() {
    cin.tie(nullptr);
    ios::sync_with_stdio(false);
    solve();
    return 0;
}