#include<map>
#include<set>
#include<list>
#include<cmath>
#include<queue>
#include<stack>
#include<cstdio>
#include<string>
#include<vector>
#include<complex>
#include<cstdlib>
#include<cstring>
#include<numeric>
#include<sstream>
#include<iostream>
#include<algorithm>
#include<functional>

#define mp       make_pair
#define pb       push_back
#define all(x)   (x).begin(),(x).end()
#define rep(i,n) for(int i=0;i<(n);i++)
#define REP(i,b,n) for(int i=b;i<n;i++)

using namespace std;

typedef    long long          ll;
typedef    unsigned long long ull;
typedef    vector<bool>       vb;
typedef    vector<int>        vi;
typedef    vector<vb>         vvb;
typedef    vector<vi>         vvi;
typedef    pair<int,int>      pii;

const int INF=1<<29;
const double EPS=1e-9;

const int dx[]={1,0,-1,0},dy[]={0,-1,0,1};
char board[1100][1100];
int dp[1100][1100];//
int main(){
	int n;
	while(cin>>n,n){
		bool fi =false;

		for(int i =1;i <=n;i++){
			for(int j =1;j <=n;j++){
				cin>>board[i][j];
				dp[i][j]=0;
			}
		}
		int ans =0;
		for(int i =1;i <=n;i++){
			for(int j =1;j <=n;j++){
				if(board[i][j]=='*'){
					dp[i][j]=0;
				}else{
					dp[i][j]=min(dp[i-1][j-1],min(dp[i-1][j],dp[i][j-1]))+1;
					if(dp[i][j]>ans){
						ans =dp[i][j];
					}
				}
			}
		}
		cout <<ans<<endl;

	}
	return 0;
}