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

using namespace std;

#define reep(i,f,n) for(int i=f; i<int(n); ++i)
#define rep(i,n) reep(i,0,n)

typedef vector<int> vi;
typedef pair<int, int> pii;
typedef complex<double> Point;

int main()
{
	int n;
	while(scanf("%d", &n), n){
		vi dp(n, 0);
		int ans = 0;
		rep(i, n){
			char field[1024];
			scanf("%s", field);
			int rt = 0;
			rep(j, n){
				int next;
				if(field[j] == '*')
					next = 0;
				else
					next = min(rt, min(dp[j], j ? dp[j-1] : 0)) + 1;
				rt = dp[j];
				dp[j] = next;
				ans = max(ans, dp[j]);
			}
		}
		printf("%d\n", ans);
	}

	return 0;
}