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
input_count = 0

N = oi()
out = set([])
dicts = defaultdict(int)
for _ in range(N):
    S = "".join(sorted(list(os())))
    dicts[S] += 1

alpha = [chr(ord("a")+i) for i in range(26)]
output = "-1"
flg = False
for o in out:
    if dicts[o]!=1:
        continue
    for a in alpha:
        if o+a not in out:
            output = o+a
            flg=True
            break
    if flg:
        break
print(output)