#include <bits/stdc++.h>
using namespace std;
#define ios ios_base::sync_with_stdio(false);cin.tie(0);
#define setnow clock_t tStart=clock();
#define time (double)(clock() - tStart)/CLOCKS_PER_SEC;
#define setin(x) ifstream cin(x);
#define setout(x) ofstream cout(x);
typedef long long ll;
typedef long long int lli;
typedef pair < int, int> dbl;
const int maxInt = 1e9*2;
const lli maxLong = 1e18*2;
map <string, bool > mp;
string names[123];
string best;
int n;
//
void check(string str, int i, int j){
		if(i == j && (unsigned) == str.size()){
				if(str.size() > best.size())
					best = str;
				return;
		}
		while((unsigned)j < str.size()){
			string sub;
			for(int k=i;k<=j;k++)
				sub+=str[k];
			if(mp[sub])
				check(str, j+1, j+1);
			j++;
		}
}
int main(){
		//ifstream cin("input.in");
		cin >> n;
		for(int i=0;i<n;i++){
				cin >> names[i];
				mp[names[i]] = true;
		}
		for(int i=0;i<n;i++){
			check(names[i],0,0);
		}
		//
		if(best.size() > 0)
			cout << best;
		else cout << -1;
		return(0);
}