#include<iostream>
#include<string>
#include<cmath>
#include<queue>
#include<map>
#include<set>
#include<list>
#include<iomanip>
#include<vector>
#include<random>
#include<functional>
#include<algorithm>
#include<stack>
#include<cstdio>
#include<bitset>
#include<unordered_map>
#include <climits>
using namespace std;
//---------------------------------------------------
//ライブラリゾーン！！！！
typedef long long ll;
typedef long double ld;
#define str string
#define rep(i,j) for(ll i=0;i<(long long)(j);i++)
#define all(a) (a).begin(),(a).end()
const ll Mod = 1000000007;
const ll gosenchou = 5000000000000000;
short gh[2][4] = { { 0,0,-1,1 },{ -1,1,0,0 } };
struct P {
	ll pos, cost;
};
bool operator<(P a, P b) { return a.cost < b.cost; }
bool operator>(P a, P b) { return a.cost > b.cost; }
struct B {//隣接リスト表現
	ll to, cost;
};
struct E {//辺の情報を入れる変数
	ll from, to, cost;
};
bool operator<(E a, E b) {
	return a.cost < b.cost;
}
struct H {
	int x, y;
};
bool operator<(H a, H b) {
	if (a.x != b.x) return a.x < b.x;
	return a.y < b.y;
}
bool operator>(H a, H b) {
	if (a.x != b.x) return a.x > b.x;
	return a.y > b.y;
}
bool operator==(H a, H b) {
	return a.x == b.x&&a.y == b.y;
}
bool operator!=(H a, H b) {
	return a.x != b.x || a.y != b.y;
}
struct LS {
	ll num, cost;
	//値、遅延配列
};
ll gcm(ll i, ll j) {//最大公約数
	if (i > j) swap(i, j);
	if (i == 0) return j;
	return gcm(j%i, i);
}
ld rad(H a, H b) {
	return sqrt(pow(a.x - b.x, 2.0) + pow(a.y - b.y, 2.0));
}//rad＝座標上の2点間の距離
ll ari(ll a, ll b, ll c) {
	return (a + b)*c / 2;
}//等差数列の和
ll fact(ll x, ll k, ll p) {//最大値、個数、あまり
	ll sum = 1;
	for (int i = 0; i < k; i++) {
		sum *= (x--);
		sum %= p;
	}
	return sum;
}//階乗(正）
ll mod_pow(ll x, ll n, ll p) {
	ll res = 1;
	while (n > 0) {
		if (n & 1) res = res * x%p;
		x = x * x%p;
		n >>= 1;
	}
	return res;
}//x^n%p
short ctos(char a) {
	return (int)(a - '0');
}
//#define int long long
const long long Inf = 4523372036854775807;
const int inf = 1500000000;
//----------------------------------------------------
//++++++++++++++++++++++++++++++++++++++++++++++++++++
int n;
int c[30];
vector<int>e[30];
signed main() {
	cin>>n;
	for(int i=0;i<n;i++){
		string s,t;
		cin>>s>>t;
		if(s==t){
			cout<<-1<<endl;
			return 0;
		}
		for(int j=0;j<t.size();j++){
			if(s[j]!=t[j]){
				int u1=(int)(s[j]-'a');
				int u2=(int)(t[j]-'a');
				e[u1].push_back(u2);
				c[u2]++;
				goto loop;
			}
		}
		cout<<-1<<endl;
		return 0;
		loop:;
	}
	string ans="";
	for(int i=0;i<26;i++){
		for(int j=0;j<26;j++){
			if(c[j]==0){
				ans+=(char)(j+'a');
				for(int z=0;z<e[j].size();z++){
					c[e[j][z]]--;
					n--;
				}
				c[j]--;
				break;
			}
		}
	}
	if(n){
		cout<<-1<<endl;
		return 0;
	}
	cout<<ans<<endl;
}
