#include "bits/stdc++.h"
using namespace std;
typedef long long ll;
typedef pair<int,int> pii;
#define rep(i,n) for(ll i=0;i<(ll)(n);i++)
#define all(a)  (a).begin(),(a).end()
#define vi vector<int>
#define pb push_back
#define INF 999999999
//#define INF (1LL<<59)


int main(){
	int h,w;
	cin>>h>>w;
	vector<string> data(h);
	rep(i,h) cin>>data[i];
	
	static int dp[3002][3002];
	rep(i,3002)rep(j,3002)dp[i][j] = -1;
	
	rep(y,h)rep(x,w)if(data[y][x]=='.')dp[y][x] = 0;
	
	int ans = 0;
	rep(y,h){
		rep(x,w){
			if(data[y][x]=='.')continue;
			
			queue<pii> que;
			stack<pii> st;
			que.push(pii(y,x));
			while(que.size()){
				pii q = que.front(); que.pop();
				st.push(q); dp[q.first][q.second]=INF;
				
				int dy[]={1,1,1,0,-1,-1,-1,0};
				int dx[]={-1,0,1,1,1,0,-1,-1};
				
				rep(i,8){
					int ddy = q.first+dy[i], ddx = q.second+dx[i];
					if(dp[ddy][ddx] == -1 && data[ddy][ddx] == '#'){
						if( ddx>=0 && ddy>=0 && ddx<w && ddy<h )que.push(pii(ddy,ddx));
					}
				}
			}
			while(st.size()){
				pii p = st.top(); st.pop();
				int mini = INF;
				int dy[]={1,1,1,0,-1,-1,-1,0};
				int dx[]={-1,0,1,1,1,0,-1,-1};
				rep(i,8){
					int ddy = p.first+dy[i], ddx = p.second+dx[i];
					if(  ddx>=0 && ddy>=0 && ddx<w && ddy<h ){
						mini = min(mini,dp[ddy][ddx]);
					}
					else mini = 0;
				}
				dp[p.first][p.second]=mini+1;
				ans = max(ans,mini+1);
			}
		}
	}
	
	cout<<ans<<endl;
}