#include <bits/stdc++.h>
const int MOD=1000000007;
const int INF=1000000000;
using namespace std;
typedef long long ll;
typedef vector<int> vi;
const double eps=1e-9;
const int inf=1e9;
typedef pair<int,int> P;
struct Point
{
  double x,y;
  Point(){x=0;y=0;}
  Point(double d_x,double d_y){x=d_x,y=d_y;}
  double operator*(Point obj){return obj.x*x+obj.y*y;}
  double operator%(Point obj){return obj.y*x-obj.x*y;}
  Point operator*(double b){Point tmp;tmp.x=x*b;tmp.y=y*b;return tmp;}
  Point operator/(double b){Point tmp;tmp.x=x/b;tmp.y=y/b;return tmp;}
  Point operator+(Point obj){Point tmp;tmp.x=x+obj.x;tmp.y=y+obj.y;return tmp;}
  Point operator-(){Point tmp;tmp.x=-x;tmp.y=-y;return tmp;}
  Point operator-(Point obj){Point tmp;tmp.x=x-obj.x;tmp.y=y-obj.y;return tmp;}
  Point operator-=(Point obj){x-=obj.x;y-=obj.y;return *this;}
  Point operator+=(Point obj){x+=obj.x;y+=obj.y;return *this;}
  Point operator/=(double b){x=x/b;y=y/b;return *this;}
  Point operator*=(double b){x=x*b;y=y*b;return *this;}
  double size(){return hypot(x,y);}
  Point unit(){return Point(x/size(),y/size());}
  Point normal(){return Point(y,-x);}
  double atan(){return atan2(y,x);}
};

bool operator<(Point a,Point b){return a.x!=b.x?a.x<b.x:a.y<b.y;}
bool operator>(Point a,Point b){return b<a;}
bool operator<=(Point a,Point b){return !(b<a);}
bool operator>=(Point a,Point b){return !(a<b);}
bool operator==(Point a,Point b){return (a-b).size()<eps;}
bool operator!=(Point a,Point b){return !(a==b);}

bool equal(double a,double b){return abs(a-b)<eps;}
double cross(Point a,Point b){return a%b;}
double dot(Point a,Point b){return a*b;}

int ccw(Point a,Point b,Point c)
{
  b=b-a;
  c=c-a;
  if(b%c>0) return +1;
  else if(b%c<0)return -1;
  else if(b*c<0) return +2;
  else if(b.size()<c.size()) return -2;
  else return 0;
}

int w,h;
vector<string> board;
vector<vector<string> > memo;
int dx[]={1,0};
int dy[]={0,1};

inline bool outrange(int x,int y)
{
  return (x<0 || x>=w || y<0 || y>=h);
}

string cmp(string a,string b)
{
  if(a.size()>b.size()) return a;
  if(b.size()>a.size()) return b;
  return max(a,b);
}

string dfs(int x,int y)
{
  if(outrange(x,y)) return "";
  if(!isdigit(board[y][x])) return "";
  if(memo[y][x]!="") return memo[y][x];
  
  string res="";
  for(int d=0;d<2;d++)
    {
      res=cmp(res,board[y][x]+dfs(x+dx[d],y+dy[d]));
    }
  memo[y][x]=res;
  return res;
}

int main(int argc,char const* argv[])
{
  string board[1005];
  int dp[1005][1005];
  int n,ans;
  while(cin >> n && n)
    {
      ans=0;
      memset(dp,0,sizeof(dp));
      for(int i=0;i<n;i++)
	{
	  cin >> board[i];
	}
      for(int i=0;i<n;i++)
	{
	  for(int j=0;j<n;j++)
	    {
	      if(board[i][j]=='*') dp[i][j]=0;
	      else
		{
		  if(i>0 && j>0)
		    {
		      dp[i][j]=min({dp[i-1][j],dp[i][j-1],dp[i-1][j-1]})+1;
		    }
		  else dp[i][j]=1;
		  ans=max(ans,dp[i][j]);
		}
	    }
	}
      cout << ans << endl;
    }
  return 0;
}

