from collections import defaultdict

n = int(input())

s = [""] * n
for i in range(n):
    s[i] = list(input())

cnt = defaultdict(int)

for i in range(n):
    for c in range(26):
        t = s[i] + [chr(ord('a') + c)]
        t.sort()
        cnt["".join(t)] += 1

for t in cnt:
    if cnt[t] == 1:
        print("".join(t))
        exit(0)
print(-1)