#include<cstring>
#include<cstdio>
const int NN = 805000;
int wa[NN], wb[NN], flag[4000], sa[NN], height[NN], rank[NN], num[NN], s[NN], w[NN], wv[NN];
char str1[4000];
int *x, *t, *y;
int p, count1, l1, xx, k, m, n, mid, l, r, ans, anss;
bool aa;
int check(int x, int f){
	int count = 0;
	memset(flag, 0, sizeof flag);
	flag[s[sa[0]]] = 1;
	for (int i = 1; i < n; i++){
		if (height[i] >= x){
			if (!flag[s[sa[i]]]) count++;
			flag[s[sa[i]]] = 1;
			
		}
		else{
			if (count >= f) {
				anss = sa[i - 1];
				return 1;
			}
			count = 0;
			memset(flag, 0, sizeof flag);
			flag[s[sa[i]]] = 1;
		}
	}
	if (count >= f) {
		anss = sa[n - 1];
		return 1;
	}
	else return 0;
}
int main(){
	freopen("poj3450.in","r", stdin);
	freopen("poj3450.out","w", stdout);
	while (1){
		scanf("%d", &xx);
		if (!xx) break;
		m = 27;
		n = 0;
		for (int i = 0; i < xx; i++){
			scanf("%s", str1);
			l1 = strlen(str1);
			for (int j = 0; j < l1; j++){
				num[n] = str1[j] - 'a' + 1;
				s[n++] = i+1;
		 	}
			num[n] = m++;
			s[n++] = 0;
		}
		//printf("%d %d\n", n, m);
		//for (int i = 0; i < n; i++) printf("%d ", num[i]);
		//printf("\n");
		//for (int i = 0; i < n; i++) printf("%d ", s[i]);
		//printf("\n");
		x = wa;
		y = wb;
		memset(w, 0, sizeof w);
		for (int i = 0; i < n; i++) w[x[i] = num[i]]++;
		for (int i = 1; i < m; i++) w[i] += w[i - 1];
		for (int i = n - 1; i >= 0; i--) sa[--w[num[i]]] = i;
		for (int j = 1; j <= n; j+=j){
			p = 0;
			for (int i = n - j; i < n; i++) y[p++] = i;
			for (int i = 0; i < n; i++) if (sa[i] >= j) y[p++] = sa[i] - j;
			for (int i = 0; i < n; i++) wv[i] = x[y[i]];
			memset(w, 0, sizeof w);
			for (int i = 0; i < n; i++) w[wv[i]]++;
			for (int i = 1; i < m; i++) w[i] += w[i - 1];
			for (int i = n - 1; i >= 0;i--) sa[--w[wv[i]]] = y[i];
			t = x;
			x = y;
			y = t;
			x[sa[0]] = 0;
			m = 1;
			for (int i = 1; i < n; i++) x[sa[i]] = (y[sa[i]] == y[sa[i - 1]] && y[sa[i] + j] == y[sa[i - 1] + j])? m - 1:m++;
			if (m == n) break;
		}
		
		for (int i = 0; i < n; i++) rank[sa[i]] = i;
		//for (int i = 0; i < n; i++) printf("%d ", sa[i]);
		//printf("\n");
		k = 0;
		for (int i = 0; i < n; i++){
			if (!rank[i]) k = 0;
			else{
				if (k) k--;
				while (num[i + k] == num[sa[rank[i] - 1] + k]) k++;
			}
			height[rank[i]] = k;
		}
		//for (int i = 0; i < n; i++) printf("%d ", height[i]);
		//printf("\n");
		//for (int i = 0; i < n; i++) printf("%d ", s[sa[i]]);
		//printf("\n");
		l = 0;
		r = n;
		ans = 0;
		while (l <= r){
			mid = (l + r) >> 1;
			if (check(mid, xx - 1)){
				ans = mid;
				l = mid + 1;
			}
			else r = mid - 1;
			//printf("%d %d\n", check(mid, xx - 1), mid);
		}
		if (ans == 0) printf("IDENTITY LOST\n");
		else{
			for (int i = anss; i < anss + ans; i++) printf("%c", num[i] - 1 + 'a');
			printf("\n");
		}
	}
	return 0;
}