include [HTML_REMOVED]
using namespace std;
string a[201];
int main()
{
int num;
while(cin >> num ,num!=0)
{
for(int i=0;i<num;i++) cin >> a[i],reverse(a[i].begin(),a[i].end());
sort(a,a+num);
string ss="";
for(int i=0;i<a[0].length();i++)
{
if(a[0][i]==a[num-1][i]) ss =a[0][i]+ss;
else break;
}
cout << ss <<endl;
}
return 0;
}