#include<iostream>
#include<string>
#include<algorithm>
using namespace std;
int main(){
int n;
cin>>n;
cin.ignore();
string ans;
for(int i=0;i<n;i++){
string s;
getline(cin,s);
reverse(s.begin(),s.end());
if(i==0){
ans=s;
continue;
}else{//让较短的字符串作为标准
if(s.length()<ans.length()){
swap(ans,s);
}
}
int minlen=min(ans.length(),s.length());
for(int j=0;j<minlen;j++){
if(ans[j]!=s[j]){
ans=ans.substr(0,j);
break;
}
}
}
reverse(ans.begin(),ans.end());
if(ans.empty()){//字符串为空
cout<<"nai"<<endl;
}else{
cout<<ans<<endl;
}
return 0;
}