#include <iostream>
#include <string>
#include <algorithm>
#include <vector>

using namespace std;

int main()
{
	int n;
	while(1)
	{
		cin>>n;
		if(n==0)
			break;
		
		vector<string> strlist;
		for(int i=0;i<n;i++)
		{
			string temp;
			cin>>temp;
			strlist.push_back(temp);
		}
		
		string minstring=strlist[0];
		string rstring="";
		
		for(int i=1;i<n;i++)
		{
			if(minstring.size()>strlist[i].size())
			 minstring=strlist[i];
		}
		
		for(int i=0;i<minstring.length();i++)
		{
			for(int j=i;j<=minstring.length()-i;j++)
			{
				string temp=minstring.substr(i,j);
				bool flag=true;
				for(int k=0;k<n;k++)
				{
					if(strlist[k].find(temp,0) == string::npos )  // not found
					{
						flag=false;
						break;
					}
				}
				if(flag)
				{
					if(j>rstring.length())
					{
						
						rstring=temp;
					}
					else if(j==rstring.length())
					{
						if(temp<rstring)
							rstring=temp;
					}
					
								
				}
			}
		}

		if(rstring=="")
			cout<<"IDENTITY LOST\n";
		else
			cout<<rstring<<endl;
		
	}
	
	return 0;
}