#include <bits/stdc++.h>
using namespace std;

int dy[] = {0, 0, 1, -1};
int dx[] = {1, -1, 0, 0};

int W, H;
string cave[30];

int id[30][30];
void dfs(int y, int x, int n){
	if(y < 0 || y >= H || x < 0 || x >= W)
		return;
	if(cave[y][x] == '#' || id[y][x] != -1)
		return;

	id[y][x] = n;
	for(int i=0;i<4;i++)
		dfs(y + dy[i], x + dx[i], n);
}

int dist[30][30];
int bfs(){
	memset(dist, -1, sizeof(dist));

	queue<pair<int,int>> Q;
	for(int y=0;y<H;y++){
		for(int x=0;x<W;x++){
			if(id[y][x] != 0)
				continue;

			dist[y][x] = 0;
			Q.push(make_pair(y, x));
		}
	}

	while(!Q.empty()){
		auto ps = Q.front(); Q.pop();
		int y = ps.first;
		int x = ps.second;
		int d = dist[y][x];

		for(int i=0;i<4;i++){
			int ny = y + dy[i];
			int nx = x + dx[i];

			if(ny < 0 || ny >= H || nx < 0 || nx >= W)
				continue;

			if(dist[ny][nx] != -1)
				continue;

			dist[ny][nx] = d + 1;
			Q.push(make_pair(ny, nx));
		}
	}

	int res = 1 << 25;
	for(int y=0;y<H;y++)
		for(int x=0;x<W;x++)
			if(id[y][x] == 1)
				res = min(res, dist[y][x]);

	return res - 1;
}

int main(){
	memset(id, -1, sizeof(id));

	cin >> W >> H;
	for(int i=0;i<H;i++)
		cin >> cave[i];

	int n = 0;
	for(int y=0;y<H;y++)
		for(int x=0;x<W;x++)
			if(cave[y][x] == '.' && id[y][x] == -1)
				dfs(y, x, n++);

	cout << bfs() << endl;
	return 0;
}