#include "bits/stdc++.h"
// Begin Header {{{
#define let             const auto
#define all(x)          (x).begin(), (x).end()
#define rep(i, n)       for (i64 i = 0, i##_limit = (n); i < i##_limit; ++i)
#define reps(i, s, t)   for (i64 i = (s), i##_limit = (t); i <= i##_limit; ++i)
#define repr(i, s, t)   for (i64 i = (s), i##_limit = (t); i >= i##_limit; --i)
#define var(Type, ...)  Type __VA_ARGS__; input(__VA_ARGS__)
#define lowerBound(...)                 lowerBound_(__VA_ARGS__)
#define upperBound(...)                 upperBound_(__VA_ARGS__)
#define lowerBound_(begin, end, ...)    (lower_bound((begin), (end), __VA_ARGS__) - (begin))
#define upperBound_(begin, end, ...)    (upper_bound((begin), (end), __VA_ARGS__) - (begin))

#ifdef DBG
#define trace(...) trace_g(#__VA_ARGS__, __VA_ARGS__)
#else
#define trace(...)
#endif

using namespace std;
using i64 = int_fast64_t;
using pii = pair<i64, i64>;
template<class T, class U>inline bool chmax(T &a, const U &b){return b>a && (a=b, true);}
template<class T, class U>inline bool chmin(T &a, const U &b){return b<a && (a=b, true);}
inline i64  sigma(i64 n)            { return (n * (n + 1) >> 1); }
inline i64  updiv(i64 a, i64 b)     { return (a + b - 1) / b; }
inline i64  sqr(i64 n)              { return n * n; }
inline string to_string(char c)     { return string(1, c); }
inline bool   isRangeIn(i64 a, i64 low, i64 high) { return (low <= a && a <= high); }
constexpr int INF  = 0x3f3f3f3f;
constexpr i64 LINF = 0x3f3f3f3f3f3f3f3fLL;

template<class T>
vector<T> makeVec(size_t sz) { return vector<T>(sz); }

template<class T, class... Args>
auto makeVec(size_t sz, Args... args) {
    return vector<decltype(makeVec<T>(args...))>(sz, makeVec<T>(args...));
}

template<class T>
inline void input(T &x) { cin >> x; }

template<class Head, class... Tail>
inline void input(Head &head, Tail&... tail) { cin >> head; input(tail...); }

inline void print() { cout << "\n"; }

template<class Head, class... Tail>
inline void print(Head &&head, Tail&&... tail) {
    cout << head;
    if (sizeof...(tail)) cout << ' ';
    print(forward<Tail>(tail)...);
}

template<class T>
ostream& operator<< (ostream &out, const vector<T> &vec) {
    static constexpr const char *delim[] = { " ", "" };
    for (const auto &e : vec) out << e << delim[&e == &vec.back()];
    return out;
}

template<class T>
ostream& operator<< (ostream &out, const vector<vector<T>> &mat) {
    static constexpr const char *tail[] = { "\n", "" };
    for (const auto &row : mat) out << row << tail[&row == &mat.back()];
    return out;
}

template <class T>
void trace_g(const char *s, T&& x) {
    clog << '{';
    while(*s != '\0') clog << *(s++);
    clog << ":" << setw(3) << x << '}' << endl;
}

template <class Head, class... Tail>
void trace_g(const char *s, Head&& head, Tail&&... tail) {
    clog << '{';
    while(*s != ',') clog << *(s++);
    clog << ":" << setw(3) << head << "}, ";
    for (++s; !isgraph(*s); ++s);
    trace_g(s, std::forward<Tail>(tail)...);
}
// }}} End Header

auto gridBFS(const vector<string> &grid, const vector<pair<int,int>> &starts, const string &walls)
    -> vector<vector<int>>
{
    static constexpr array<int, 4> dy {1, 0, -1, 0};
    static constexpr array<int, 4> dx {0, 1, 0, -1};
    static_assert(dy.size() == dx.size(), "Length of dy[], dx[] is different!");

    const int H = grid.size();
    const int W = grid[0].size();
    vector<vector<int>> dist(H, vector<int>(W, -1));
    queue<pair<int,int>> que;

    for (const auto &e : starts) {
        dist[e.first][e.second] = 0;
        que.emplace(e.first, e.second);
    }

    while(!que.empty()) {
        const int curY = que.front().first;
        const int curX = que.front().second;
        que.pop();
        for (int i = 0; i < dy.size(); ++i) {
            const int ny = curY + dy[i];
            const int nx = curX + dx[i];
            if (ny < 0 || nx < 0 || ny >= H || nx >= W) continue;
            if (dist[ny][nx] != -1) continue;
            if (walls.find(grid[ny][nx]) != string::npos) continue;
            dist[ny][nx] = dist[curY][curX] + 1;
            que.emplace(ny, nx);
        }
    }

    return dist;
}

signed main()
{
    ios::sync_with_stdio(false); cin.tie(nullptr);

    var(int, H, W);
    vector<string> mat(H);
    vector<pair<int,int>> starts;

    rep(i, H) {
        input(mat[i]);
        rep(j, W) {
            if (mat[i][j] == '#') starts.emplace_back(i, j);
        }
    }

    const auto dist = gridBFS(mat, starts, "#");

    int res = 0;
    rep(i, H) rep(j, W) chmax(res, dist[i][j]);

    print(res);

    return 0;
}


Measured memory_bytes: 21224