#include <bits/stdc++.h>
using namespace std;
int main () {
	int H, W;
	cin >> H >> W;
	int A[4040][4040];
	char C[2020][2020];
	for (int i = 0; i < H; i ++) {
		for (int j = 0; j < W; j ++) {
			char c;
			cin >> c;
			A[i + 1][j + 1] = c == '#';
			C[i][j] = c;
		}
	}
	int ng = min(H, W) + 1, ok = 1;
	for (int i = 0; i <= H + ng; i ++) {
		for (int j = 0; j <= W + ng; j ++) {
			if (0 < i && i <= H && 0 < j && j <= W) continue;
			A[i][j] = 0;
		}
	}
	for (int i = 0; i < H + ng; i ++) {
		for (int j = 1; j < W + ng; j ++) {
			A[i][j] += A[i][j - 1];
		}
	}
	for (int j = 0; j < W + ng; j ++) {
		for (int i = 1; i < H + ng; i ++) {
			A[i][j] += A[i - 1][j];
		}
	}
	int B[2040][2040];
	while (abs(ok - ng) > 1) {
		int mu = (ok + ng) / 2;
		for (int i = 0; i < H; i ++) {
			for (int j = 0; j < W; j ++) {
				B[i][j] = 0;
			}
		}
		for (int i = 0; i < H; i ++) {
			for (int j = 0; j < W; j ++) {
				if (A[i][j] + A[i + mu][j + mu] - A[i][j + mu] - A[i + mu][j] >= mu * mu) {
					B[i][j] ++;
					B[i + mu][j] --;
					B[i][j + mu] --;
					B[i + mu][j + mu] ++;
				}
			}
		}
		for (int i = 0; i < H; i ++) {
			for (int j = 1; j < W; j ++) {
				B[i][j] += B[i][j - 1];
			}
		}
		for (int j = 0; j < W; j ++) {
			for (int i = 1; i < H; i ++) {
				B[i][j] += B[i - 1][j];
			}
		}
		bool cn = true;
		for (int i = 0; i < H; i ++) {
			for (int j = 0; j < W; j ++) {
				if (C[i][j] == '#') {
					cn = cn && B[i][j] > 0;
				}
			}
		}
		if (cn) {
			ok = mu;
		} else {
			ng = mu;
		}
	}
	cout << ok << endl;
}