#include "bits/stdc++.h"
using namespace std;
#define rep(i,n) for(int (i)=0;(i)<(int)(n);++(i))
#define rer(i,l,u) for(int (i)=(int)(l);(i)<=(int)(u);++(i))
#define reu(i,l,u) for(int (i)=(int)(l);(i)<(int)(u);++(i))
static const int INF = 0x3f3f3f3f; static const long long INFL = 0x3f3f3f3f3f3f3f3fLL;
typedef vector<int> vi; typedef pair<int, int> pii; typedef vector<pair<int, int> > vpii; typedef long long ll;
template<typename T, typename U> static void amin(T &x, U y) { if(y < x) x = y; }
template<typename T, typename U> static void amax(T &x, U y) { if(x < y) x = y; }


class FastInput {
	bool _end;
public:
	FastInput() : _end(false) {}

	operator void*() { return _end ? 0 : (void*)this; }

	template<typename T>
	void read_unsigned(T *res) {
		T x = 0;
		for(char c = skip(); '0' <= c && c <= '9'; c = gc())
			x = x * 10 + (c - '0');
		*res = x;
	}

	template<typename T>
	void read_signed(T *res) {
		char c = skip();
		bool sign = false;
		if(c == '-') sign = true, c = gc();
		T x = 0;
		for(; '0' <= c && c <= '9'; c = gc())
			x = x * 10 + (c - '0');
		*res = !sign ? x : -x;
	}

	void read_c_string(char *str, int *len) {
		int n = 0;
		for(char c = skip(); !is_delim(c); c = gc())
			str[n ++] = c;
		str[n] = 0;
		*len = n;
	}

	void read_string(std::string *str) {
		str->clear();
		for(char c = skip(); !is_delim(c); c = gc())
			*str += c;
	}

	void read_line(std::string *str) {
		str->clear();
		for(char c = gc(); c != '\n'; c = gc())
			*str += c;
		if(!str->empty() && (*str)[str->size() - 1] == '\r') str->resize(str->size() - 1);
	}

	void read_double(double *res) {
		std::string buf;
		read_string(&buf);
		sscanf(buf.c_str(), "%lf", res);
	}

	void read_char(char *res) {
		*res = skip();
	}

	void read_string_buf(char *res, int n) {
		*res = skip();
	}

	FastInput &operator()(char &res) { read_char(&res); return *this; }
	FastInput &operator()(int &res) { read_signed(&res); return *this; }
	FastInput &operator()(unsigned &res) { read_unsigned(&res); return *this; }
	FastInput &operator()(long long &res) { read_signed(&res); return *this; }
	FastInput &operator()(unsigned long long &res) { read_unsigned(&res); return *this; }
	FastInput &operator()(char *res) { int len; read_c_string(res, &len); return *this; }
	FastInput &operator()(std::string &res) { read_string(&res); return *this; }
	FastInput &operator()(double &res) { read_double(&res); return *this; }

	template<typename T1, typename T2>
	FastInput &operator()(T1 &res1, T2 &res2) {
		return operator()(res1)(res2);
	}
	template<typename T1, typename T2, typename T3>
	FastInput &operator()(T1 &res1, T2 &res2, T3 &res3) {
		return operator()(res1)(res2)(res3);
	}

	template<typename T>
	FastInput &a(T *a, int n) {
		for(int i = 0; i < n; ++ i)
			operator()(a[i]);
		return *this;
	}

	template<typename T>
	FastInput &operator()(vector<T> &v) {
		for(size_t i = 0; i < v.size(); ++ i)
			operator()(v[i]);
		return *this;
	}

private:
	static char gc() {
#if defined(__GNUC__) && !defined(__MINGW32__)
		return (char)getchar_unlocked();
#elif defined(_MSC_VER)
		return (char)_getchar_nolock();
#else
		return (char)getchar();
#endif
	}

	static bool is_delim(char c) {
		return c == ' ' || c == '\t' || c == '\n' || c == '\r' || c == EOF;
	}

	char skip() {
		if(_end) return EOF;
		char c;
		for(c = gc(); c != -1 && is_delim(c); c = gc());
		if(c == EOF) _end = true;
		return c;
	}
} in;

int main() {
	int H; int W;
	while(in(H, W)) {
		vector<unique_ptr<char[]>> S(H);
		rep(i, H) {
			S[i] = make_unique<char[]>(W + 2);
			in(S[i].get());
		}
		vector<vector<int>> dp(H + 1, vector<int>(W + 1));
		int m = 0;
		rep(i, H) rep(j, W) if(S[i][j] == '#') {
			int x = min({ dp[i][j + 1], dp[i + 1][j], dp[i][j] }) + 1;
			amax(m, x);
			dp[i + 1][j + 1] = x;
		}
		int ans = (m + 1) / 2;
		printf("%d\n", ans);
	}
	return 0;
}