#include <bits/stdc++.h>
#define pb push_back
#define NMAX 105
#define ll long long
#define x first
#define y second
//#define cout fout

using namespace std;

ofstream fout("fisier.in");

int n,okg;
string cuv[NMAX],ans;

void bkt(int who, int pos) {
	if(pos==cuv[who].size()-1) {
		// watch out
		if(ans.size()<cuv[who].size()) ans=cuv[who];
		okg=0;
	}
	for(int i=1;i<=n;++i) {
		bool ok=1;

		if(i==who) continue;
		for(int j=0;j<cuv[i].size();++j) {
			if(cuv[who][pos+j+1]!=cuv[i][j]) {
				ok=0;
				break;
			}

		}

		if(ok && okg) bkt(who,pos+cuv[i].size());
	}
}

int main() {
	int i,j;

	cin>>n;
	for(i=1;i<=n;++i) cin>>cuv[i];

	for(i=1;i<=n;++i) {
		okg=1;
		bkt(i,-1);
	}

	if(ans.size()) cout<<ans;
	else cout<<-1;

	return 0;
}