#define _CRT_SECURE_NO_WARNINGS
#define _USE_MATH_DEFINES
#include <cfloat>
#include <climits>
#include <cmath>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <ctime>
#include <iostream>
#include <memory>
#include <string>

#include <algorithm>
#include <complex>
#include <list>
#include <map>
#include <numeric>
#include <queue>
#include <set>
#include <stack>
#include <vector>
#include <bitset>

using namespace std;

#ifdef _MSC_VER
#define __typeof__ decltype
#define strtok_r strtok_s
#endif

#define ITER(c)             __typeof__((c).begin())
#define FOREACH(it, c)      for (ITER(c) it=(c).begin(); it != (c).end(); ++it)
#define RITER(c)            __typeof__((c).rbegin())
#define RFOREACH(it, c)     for (RITER(c) it=(c).rbegin(); it != (c).rend(); ++it)
#define REP(i, n)           REPEAT(i, 0, n)
#define RREP(i, n)          RREPEAT(i, 0, n)
#define REPEAT(i, k, n)     for(int i = (k); i < (k+n); ++i)
#define RREPEAT(i, k, n)    for(int i = (k)+(n)-1; i >= (k); --i)
#define ALL(c)              (c).begin(), (c).end()
#define LLPOW(p1,p2)        ((ll)pow((double)(p1), (int)p2))
#define ESIZEOF(A)          (sizeof(A)/sizeof((A)[0]))
#define CIN_NO_SYNC         do { cin.tie(0); ios_base::sync_with_stdio(false); } while(0)
#define GETSTR(p)           fgets((p), sizeof(p), stdin)
#define CHOMP(p)            do{ char *_q = (p) + strlen(p)-1; if(*_q == '\n' || *_q == '\r') *_q = 0; } while(0)
#define FILL(m,v)           memset(m, v, sizeof(m))
#define mp                  make_pair
#define pb                  push_back
template<class _T> _T MAX(_T p1,_T p2){return (p1>p2)?p1:p2;} 
template<class _T> _T MIN(_T p1,_T p2){return (p1<p2)?p1:p2;} 

typedef long long ll;
typedef unsigned long long ull;

#define X real()
#define Y imag()
typedef double D;
typedef complex<D> P;
#define EPS (1e-9)

#ifdef _DEBUG
template<class _T>
inline void _prfe(const char *_n, _T _c, bool _p = false){
    ITER(_c) _it=_c.begin();
    if(_p){ cout<<_n<<" = ["<<endl;for(;_it!=_c.end();++_it)cout<<"    "<<*_it<<endl;cout<<"]"<<endl; }
    else{ cout<<_n<<" = [ "<<*_it++;for(;_it!=_c.end();++_it)cout<<", "<<*_it;cout<<" ]"<<endl; }
}
#define pf printf
#define pr(n) do{cout<<#n" = "<<(n)<<endl;}while(0)
#define prfe(n) _prfe(#n, n)
#define prfep(n) _prfe(#n, n, true)
#else
#define pf(...) /* */
#define pr(...) /* */
#define prfe(...) /* */
#define prfep(...) /* */
#endif

int t[1010][1010];

void solve(int n, vector<string> &v){
    int max = 0;
    REP(y,n){
        string &s = v[y];
        REP(x,n){
            if(s[x] == '*')
                t[y][x] = 0;
            else {
                int min = 1010;
                min = MIN(min, (x>0?t[y][x-1]:0));
                min = MIN(min, (y>0?t[y-1][x]:0));
                min = MIN(min, (x>0&&y>0?t[y-1][x-1]:0));
                min++;
                t[y][x] = min;
                if(max < min) max = min;
            }
        }
    }
    cout << max << endl;
}

int main(){
    int n;
    while(cin >> n){
        if(!n) break;
        vector<string> v;
        string s;
        REP(i,n){
            cin >> s;
            v.pb(s);
        }
        solve(n, v);
    }
    return 0;
}

Measured memory_bytes: 6068