import sys
input = sys.stdin.readline
from collections import *

def strsort(s):
    s = sorted(s)
    return "".join(s)

N = int(input())
D = defaultdict(int)
S = [None] * N
for i in range(N):
    S[i] = input().rstrip()
    S[i] = strsort(S[i])
    for k in range(26):
        temp = S[i] + chr(ord("a") + k)
        temp = strsort(temp)
        D[temp] += 1
    
for i in range(N):
    for k in range(26):
        temp = S[i] + chr(ord("a") + k)
        temp = strsort(temp)
        if D[temp] == 1:
            print(temp)
            exit()
print(-1)