from collections import *

N = int(input())
S = [None] * N
for i in range(N):
    S[i] = Counter(input())
    
def check(a, b):
    for k, v in S[b].items():
        if S[a][k] < v:
            return 0
    return 1
    
for i in range(N):
    temp = 0
    flag = 1
    for j in range(N):
        if i == j:
            continue
        if S[i] == S[j]:
            flag = 0
            break
    if not flag:
        continue
    
    for k in range(26):
        S[i][chr(ord("a") + k)] += 1
        for j in range(N):
            if check(i, j):
                temp += 1
        if temp == 1:
            ans = ""
            for k, v in S[i].items():
                ans += k * v
            print(ans)
            exit()
        S[i][chr(ord("a") + k)] -= 1
        
print(-1)