#include <iostream>
#include <iomanip>
#include <string>
#include <cmath>
#include <algorithm>
#include <vector>
#include <set>
#include <map>
#include <unordered_map>
#include <unordered_set>
#include <list>
#include <stack>
#include <queue>
#include <bitset>
#include <numeric>

/* (=＾o＾=) */
#define int ll

/* macro */
#define FOR(i, b, e) for(ll i = (ll)(b); i < (ll)(e); ++i)
#define RFOR(i, b, e) for(ll i = (ll)(e-1); i >= (ll)(b); --i)
#define REP(i, n) FOR(i, 0, n)
#define RREP(i, n) RFOR(i, 0, n)
#define REPC(x,c) for(const auto& x:(c))
#define REPI2(it,b,e) for(auto it = (b); it != (e); ++it)
#define REPI(it,c) REPI2(it, (c).begin(), (c).end())
#define RREPI(it,c) REPI2(it, (c).rbegin(), (c).rend())
#define REPI_ERACE2(it, b, e) for(auto it = (b); it != (e);)
#define REPI_ERACE(it, c) REPI_ERACE2(it, (c).begin(), (c).end())
#define ALL(x) (x).begin(),(x).end()
#define cauto const auto&
/* macro func */
#define SORT(x) do{sort(ALL(x));}while(false)
#define RSORT(x) do{sort((x).rbegin(),(x).rend());}while(false)
#define UNIQUE(v) do{v.erase( unique(v.begin(), v.end()), v.end() );}while(false)
#define MAX(x,y) do{x = std::max(x,y);}while(false)
#define MIN(x,y) do{x = std::min(x,y);}while(false)
#define BR do{cout<<"\n";}while(false)
#define dump(...) do{ auto __DUMP_NAME_LIST__ = split(#__VA_ARGS__,','); print(__DUMP_NAME_LIST__, __VA_ARGS__);BR;}while(false)

/* type define */
using ll = long long;
using PAIR = std::pair<ll, ll>;
using VS = std::vector<std::string>;
using VL = std::vector<long long>;
using VVL = std::vector<VL>;
using VVVL = std::vector<VVL>;
using VD = std::vector<double>;

/* using std */
using std::cout;
constexpr char endl = '\n';
using std::cin;
using std::sort;
using std::pair;
using std::string;
using std::stack;
using std::queue;
using std::vector;
using std::list;
using std::map;
using std::unordered_map;
using std::multimap;
using std::unordered_multimap;
using std::set;
using std::unordered_set;
using std::unordered_multiset;
using std::multiset;
using std::bitset;
using std::priority_queue;

/* constant value */
constexpr ll MOD = 1000000007;
//constexpr ll MOD = 998244353;

/* Initial processing  */
struct Preprocessing { Preprocessing() { std::cin.tie(0); std::ios::sync_with_stdio(0); }; }_Preprocessing;

/* define hash */
namespace std { template <>	class hash<std::pair<ll, ll>> { public:	size_t operator()(const std::pair<ll, ll>& x) const { return hash<ll>()(1000000000 * x.first + x.second); } }; }

/* input output */
template<class T> std::istream& operator >> (std::istream& is, vector<T>& vec) { for (T& x : vec) is >> x; return is; }
template<class S, class T>std::ostream& operator<<(std::ostream & os, const std::pair<S, T> & p) { os << "(" << p.first << ", " << p.second << ")"; return os; }

/* for dump function */
inline std::list<std::string> split(std::string str, char del) { std::list<std::string> sList; string s = ""; for (const auto& c : str) { if (c == del) { sList.emplace_back(s); s = ""; } else { if (c != ' ' || del == ' ') { s += c; } } }sList.emplace_back(s);	return sList; }
template<class T>struct has_begin { private:	template <class U>	static auto check(U x) -> decltype(x.begin(), std::true_type{});	static std::false_type check(...); public:	static bool const value = decltype(check(std::declval<T>()))::value; };
inline void print(std::list<std::string> & str); template<class Primitive, class... Tail, std::enable_if_t<!has_begin<Primitive>::value, std::nullptr_t> = nullptr>inline void print(std::list<std::string> & str, const Primitive & x, const Tail & ... tail); template<class Container, class... Tail>inline auto print(std::list<std::string> & str, const Container & c, const Tail & ... tail) -> decltype(c.begin()); template<class... Tail>inline void print(std::list<std::string> & str, const std::string & s, const Tail & ... tail);
template<class Container>inline auto printSingle(const Container & c) ->decltype(c.begin()) { for (const auto& x : c) { std::cout << x << " "; }std::cout << "\n"; return c.begin(); }
template<class Primitive, std::enable_if_t<!has_begin<Primitive>::value, std::nullptr_t> = nullptr>inline void printSingle(const Primitive & x) { std::cout << x << " "; }
inline void print(std::list<std::string> & str) {}
template<class Primitive, class... Tail, std::enable_if_t<!has_begin<Primitive>::value, std::nullptr_t> = nullptr>inline void print(std::list<std::string> & str, const Primitive & x, const Tail & ... tail) { std::cout << *str.begin() << ":" << x << " "; if (sizeof...(tail) > 0) { std::cout << "\n"; str.pop_front(); print(str, tail...); } }
template<class Container, class... Tail>inline auto print(std::list<std::string> & str, const Container & c, const Tail & ... tail) ->decltype(c.begin()) { std::cout << "-- " << *str.begin() << " --\n"; for (const auto& x : c) { printSingle(x); BR; }std::cout << "\n"; str.pop_front();	print(str, tail...); return c.begin(); }
template<class... Tail>inline void print(std::list<std::string> & str, const std::string & s, const Tail & ... tail) { std::cout << *str.begin() << ":" << s << "\n"; str.pop_front();	print(str, tail...); }

//=============================================================================================

signed main() {
	ll h, w;
	cin >> h >> w;
	VS map(h);
	cin >> map;

	using TYPE = pair<ll, PAIR>;
	priority_queue<TYPE, vector<TYPE>, std::greater<TYPE>> q;
	REP(i, h)REP(j, w) {
		if (map[i][j] == '#') { q.emplace(0, PAIR(i, j)); };
	}

	VVL ans(h, VL(w, 1e10));

	auto update = [&](ll y, ll x, ll val) {
		if (y < 0 || x < 0 || y >= h || x >= w) { return false; }
		if (val < ans[y][x]) {
			// ans[y][x] = val;
			return true;
		}
		return false;
	};

	while (!q.empty()) {
		ll y = q.top().second.first;
		ll x = q.top().second.second;
		ll val = q.top().first;
		// dump(y, x, val); BR;
		q.pop();
		if (val < ans[y][x]) {
			ans[y][x] = val;
		} else {
			continue;
		}
		if (update(y + 1, x, val + 1)) { q.emplace(val + 1,PAIR(y + 1, x)); }
		if (update(y - 1, x, val + 1)) { q.emplace(val + 1,PAIR(y - 1, x)); }
		if (update(y, x + 1, val + 1)) { q.emplace(val + 1,PAIR(y, x + 1)); }
		if (update(y, x - 1, val + 1)) { q.emplace(val + 1,PAIR(y, x - 1)); }
	}
	// dump(ans);
	ll tmp = 0;
	REPC(y, ans)REPC(x, y) {
		MAX(tmp, x);
	}
	cout << tmp << endl;
}


Measured memory_bytes: 59748