#include<bits/stdc++.h>
using namespace std;
int n;
string a[209];
int main(){
    while(cin>>n){
        if(n==0) return 0;
        for(int i=0;i<n;++i) cin>>a[i],reverse(a[i].begin(),a[i].end());
        sort(a,a+n);
        string v="";
        for(int i=0;i<a[0].length();++i){
            if(a[0][i]==a[n-1][i]) v=a[0][i]+v;
            else break;
        }
        cout<<v<<"\n";
    }
}