from collections import defaultdict
n = int(input())

d = defaultdict(int)

for _ in range(n):
    L = [0]*26
    for si in input():
        x = ord(si) - ord("a")
        L[x] += 1
    d[tuple(L)] += 1

for k in d.keys():
    if d[k] == 1:
        ans = ""
        for i,ai in enumerate(k):
            if ai > 0:
                sai = chr(i+ord("a"))
                ans += sai
        ans += "a"
        print(ans)
        exit()
print("-1")