N = int(input())
S = [input() for _ in range(N)]

cnt = [[0]*26 for _ in range(N)]
for i in range(len(S)):
    for j in range(len(S[i])):
        cnt[i][ord(S[i][j])-ord("a")] += 1

def check(C, A):
    flag = False
    for i in range(26):
        if C[i] != A[i]:
            if flag:
                return False
            elif C[i]+1 == A[i]:
                flag = True
            else:
                return False
    return flag

for i in range(N):
    A = cnt[i][:]
    for j in range(26):
        A[j] += 1
        c = 0
        for k in range(N):
            if len(S[i]) == len(S[k]) and check(cnt[k], A):
                c += 1
            if c >= 2:
                break
        if c == 1:
            exit(print(S[i] + chr(j+ord("a"))))
        A[j] -= 1

print(-1)