#include <iostream>
#include <vector>
#include <string>
#include <algorithm>

using namespace std;

class Cum2 {
public:
    vector<vector<int>> cum;

    Cum2(const vector<vector<int>>& A) {
        int H = A.size();
        int W = A[0].size();
        cum = vector<vector<int>>(H + 1, vector<int>(W + 1, 0));

        for (int i = 0; i < H; ++i) {
            for (int j = 0; j < W; ++j) {
                cum[i + 1][j + 1] = A[i][j];
            }
        }

        for (int i = 1; i <= H; ++i) {
            for (int j = 2; j <= W; ++j) {
                cum[i][j] += cum[i][j - 1];
            }
        }

        for (int i = 2; i <= H; ++i) {
            for (int j = 1; j <= W; ++j) {
                cum[i][j] += cum[i - 1][j];
            }
        }
    }

    int query(int lh, int lw, int rh, int rw) {
        return cum[rh + 1][rw + 1] + cum[lh][lw] - cum[rh + 1][lw] - cum[lh][rw + 1];
    }
};

int H, W;
vector<string> S;

bool func(int n, Cum2& cum) {
    vector<vector<int>> imos(H + 1, vector<int>(W + 1, 0));

    for (int i = 0; i <= H - n; ++i) {
        for (int j = 0; j <= W - n; ++j) {
            if (cum.query(i, j, i + n - 1, j + n - 1) == n * n) {
                imos[i][j] += 1;
                imos[i + n][j + n] += 1;
                imos[i][j + n] -= 1;
                imos[i + n][j] -= 1;
            }
        }
    }

    for (int i = 0; i <= H; ++i) {
        for (int j = 1; j <= W; ++j) {
            imos[i][j] += imos[i][j - 1];
        }
    }

    for (int i = 1; i <= H; ++i) {
        for (int j = 0; j <= W; ++j) {
            imos[i][j] += imos[i - 1][j];
        }
    }

    for (int i = 0; i < H; ++i) {
        for (int j = 0; j < W; ++j) {
            if (imos[i][j] >= 1) {
                if (S[i][j] != '#') return false;
            } else {
                if (S[i][j] != '.') return false;
            }
        }
    }
    return true;
}

int main() {
    cin >> H >> W;
    S.resize(H);
    for (int i = 0; i < H; ++i) {
        cin >> S[i];
    }

    vector<vector<int>> C(H, vector<int>(W, 0));
    for (int i = 0; i < H; ++i) {
        for (int j = 0; j < W; ++j) {
            if (S[i][j] == '#') {
                C[i][j] = 1;
            }
        }
    }

    Cum2 cum(C);

    int left = 1, right = min(H, W) + 1;
    while (left + 1 < right) {
        int mid = (left + right) / 2;
        if (func(mid, cum)) {
            left = mid;
        } else {
            right = mid;
        }
    }

    cout << left << endl;

    return 0;
}