#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; }