from collections import Counter
from string import ascii_lowercase


def main():
    N = int(input())
    S = [input() for _ in range(N)]
    S_ctrs = list(map(Counter, S))

    S_sorted = list(map(lambda elm: "".join(sorted(elm)), S))
    S_cand = set()
    S_sorted_ctr = Counter(S_sorted)
    for sort_elm, elm in zip(S_sorted, S):
        if S_sorted_ctr[sort_elm] == 1:
            S_cand.add(elm)

    if not S_cand:
        print(-1)
        return

    for elm in S_cand:
        for add_letter in ascii_lowercase:
            tmp_ctr = Counter(elm)
            tmp_ctr[add_letter] += 1
            for other_elm, other_ctr in zip(S, S_ctrs):
                if other_elm == elm:
                    continue
                tmp_ctr.subtract(other_ctr)
                tmp_ctr_ctr = Counter(tmp_ctr.values())
                if set(tmp_ctr_ctr).issubset({0, 1}) and tmp_ctr_ctr[1] == 1:
                    break
            else:
                print(elm + add_letter)
                return
    print(-1)


if __name__ == "__main__":
    main()