#include<iostream>
#include<stdio.h>
#include<string.h>
using namespace std;

#define M 40005
#define N 210

char s[M][N];
int Next[N];

void FindNext(char b[])
{
    int i=0, j=-1, blen=strlen(b);
    Next[0] = -1;

    while(i<blen)
    {
        if(j==-1 || b[i]==b[j])
            Next[++i] = ++j;
        else
            j = Next[j];
    }
}

int KMP(char a[], char b[])
{
    int i=0, j=0;
    int alen=strlen(a), blen=strlen(b);

    FindNext(b);

    while(i<alen)
    {
        while(j==-1 || (a[i]==b[j] && i<alen && j<blen))
            i++, j++;
        if(j==blen)
            return 1;
        j = Next[j];
    }
    return 0;
}

int main()
{
    int n;
    while(scanf("%d", &n), n)
    {
        int i, j, k, MinLen=1000, len;
        char ss[N];

        memset(s, 0, sizeof(s));

        for(i=0; i<n; i++)
        {
            scanf("%s", s[i]);
            len = strlen(s[i]);

            if(len<MinLen)
            {
                MinLen = len;
                memset(ss, 0, sizeof(ss));
                strcpy(ss, s[i]);
            }
        }

        char b[N]="{";
        int index=0;

        for(i=MinLen; i>0; i--)
        {
            for(j=0; j<=MinLen-i; j++)
            {
                char a[N];

                memset(a, 0, sizeof(a));

                strncpy(a, ss+j, i);

                for(k=0; k<n; k++)
                {
                    if(KMP(s[k], a)==0)
                        break;
                }

                if(k==n && strcmp(a, b)<0)
                {
                    index = i;
                    strcpy(b, a);
                }

                if(index && j==MinLen-i)
                    i=-1, j=1000;
            }
        }

        if(index)
            printf("%s\n", b);
        else
            printf("IDENTITY LOST\n");

    }
    return 0;
}