1 #include <iostream>
  2 #include <cstdio>
  3 #include <cstring>
  4 #include <algorithm>
  5 #include <vector>
  6 #include <cmath>
  7 #include <queue>
  8 #include <stack>
  9 #include <map>
 10 #include <string>
 11 #include <set>
 12 using namespace std;
 13 typedef long long LL;
 14 const int INF = 2e9;
 15 const LL LNF = 9e18;
 16 const int MOD = 1e9+7;
 17 const int MAXN = 1e6+100;
 18
 19 int id[MAXN];   //记录属于哪个字符串
 20 int r[MAXN], sa[MAXN], Rank[MAXN], height[MAXN];
 21 int t1[MAXN], t2[MAXN], c[MAXN];
 22
 23 bool cmp(int *r, int a, int b, int l)
 24 {
 25     return r[a]==r[b] && r[a+l]==r[b+l];
 26 }
 27
 28 void DA(int str[], int sa[], int Rank[], int height[], int n, int m)
 29 {
 30     n++;
 31     int i, j, p, *x = t1, *y = t2;
 32     for(i = 0; i<m; i++) c[i] = 0;
 33     for(i = 0; i<n; i++) c[x[i] = str[i]]++;
 34     for(i = 1; i<m; i++) c[i] += c[i-1];
 35     for(i = n-1; i>=0; i--) sa[--c[x[i]]] = i;
 36     for(j = 1; j<=n; j <<= 1)
 37     {
 38         p = 0;
 39         for(i = n-j; i<n; i++) y[p++] = i;
 40         for(i = 0; i<n; i++) if(sa[i]>=j) y[p++] = sa[i]-j;
 41
 42         for(i = 0; i<m; i++) c[i] = 0;
 43         for(i = 0; i<n; i++) c[x[y[i]]]++;
 44         for(i = 1; i<m; i++) c[i] += c[i-1];
 45         for(i = n-1; i>=0; i--) sa[--c[x[y[i]]]] = y[i];
 46
 47         swap(x, y);
 48         p = 1; x[sa[0]] = 0;
 49         for(i = 1; i<n; i++)
 50             x[sa[i]] = cmp(y, sa[i-1], sa[i], j)?p-1:p++;
 51
 52         if(p>=n) break;
 53         m = p;
 54     }
 55
 56     int k = 0;
 57     n--;
 58     for(i = 0; i<=n; i++) Rank[sa[i]] = i;
 59     for(i = 0; i<n; i++)
 60     {
 61         if(k) k--;
 62         j = sa[Rank[i]-1];
 63         while(str[i+k]==str[j+k]) k++;
 64         height[Rank[i]] = k;
 65     }
 66 }
 67
 68 bool vis[4040];
 69 int Le, Ri;
 70 bool test(int n, int len, int k)
 71 {
 72     int cnt = 0;
 73     memset(vis, false, sizeof(vis));
 74     for(int i = 2; i<=len; i++)
 75     {
 76         if(height[i]<k)
 77         {
 78             cnt = 0;
 79             memset(vis, false, sizeof(vis));
 80         }
 81         else
 82         {
 83             if(!vis[id[sa[i-1]]]) vis[id[sa[i-1]]] = true, cnt++;
 84             if(!vis[id[sa[i]]]) vis[id[sa[i]]] = true, cnt++;
 85             if(cnt==n)
 86             {
 87                 Le = sa[i]; Ri = sa[i]+k-1;
 88                 return true;
 89             }
 90         }
 91     }
 92     return false;
 93 }
 94
 95 char str[MAXN];
 96 int main()
 97 {
 98     int n;
 99     while(scanf("%d", &n)&&n)
100     {
101         int len = 0;
102         for(int i = 0; i<n; i++)
103         {
104             scanf("%s", str);
105             int LEN = strlen(str);
106             for(int j = 0; j<LEN; j++)
107             {
108                 r[len] = str[j]-'a'+1;
109                 id[len++] = i;
110             }
111             r[len] = 30+i;  //分隔符要各异
112             id[len++] = i;
113         }
114         r[len] = 0;
115         DA(r,sa,Rank,height,len,30+n);
116
117         int L = 0, R = strlen(str);
118         while(L<=R)
119         {
120             int mid = (L+R)>>1;
121             if(test(n,len,mid))
122                 L = mid + 1;
123             else
124                 R = mid - 1;
125         }
126
127         if(R==0) puts("IDENTITY LOST");
128         else
129         {
130             for(int i = Le; i<=Ri; i++)
131                 printf("%c", r[i]+'a'-1);
132             putchar('\n');
133         }
134     }
135 }