def oi(): return int(input())
def os(): return input().rstrip()
def mi(): return list(map(int, input().split()))

# import sys
# input = sys.stdin.readline
# import sys
# sys.setrecursionlimit(10**8)
# import pypyjit
# pypyjit.set_param('max_unroll_recursion=-1')
from collections import defaultdict, Counter
input_count = 0

N = oi()
dicts = defaultdict(int)
dicts_counter = defaultdict(dict)
out = []
for _ in range(N):
    S = "".join(sorted(list(os())))
    dicts[S] += 1
    dicts_counter[S] = Counter(S)
# 包含関係を確認する
OK = []

# Aの中にBが含まれていたらTrueを返す
def check(A, B, As, Bs):
    # As = Counter(A)
    # Bs = Counter(B)

    keys = set(list(As.keys()) + list(Bs.keys()))
    
    if len(A) < len(B):
        for k in keys:
            if k not in Bs and k in As:
                return False

            if k in As and k in Bs and As[k] > Bs[k]:
                return False

    elif len(A) > len(B):
        for k in keys:
            if k in Bs and k not in As:
                return False

            if k in As and k in Bs and As[k] < Bs[k]:
                return False
    else:
        if set(list(As.keys())) != set(list(Bs.keys())):
            return False
        
        for k in keys:
            if As[k] != Bs[k]:
                return False
            
    return True
OK_list = []
keys = list(dicts.keys())
for i in range(len(keys)):
    o1 = keys[i]
    flg = True
    for j in range(i+1, len(keys)):
        o2 = keys[j]
        if check(o1, o2, dicts_counter[o1], dicts_counter[o2]):
            flg = False
            break
        
    if flg:
        OK_list.append(o1)
alpha = [chr(ord("a")+i) for i in range(26)]
output = "-1"
flg = False
for o in OK_list:
    for a in alpha:
        temp = o+a
        count = 0
        for o1 in dicts.keys():
            if check(temp, o1, Counter(temp), dicts_counter[o1]):
                count += dicts[o1]
        
        if count == 1:
            output = temp
            flg=True
            break
    if flg:
        break
print(output)