N = int(input())
S = [tuple(sorted(list(input()))) for _ in range(N)]
C = {}
for i in range(N):
    if S[i] not in C:
        C[S[i]] = []
    C[S[i]].append(i)
L = {}
for s in C:
    if len(C[s])>1:continue
    i = C[s][0]
    if len(S[i]) not in L:
        L[len(S[i])] = []
    L[len(S[i])].append(i)
def dist(x,y):
    cnt = 0
    for i in range(len(x)):
        if x[i]!=y[i]:
            cnt += 1
    return cnt
Let = set([chr(i) for i in range(97,123)])
T = ""
for l in L:
    for i in L[l]:
        A = set()
        for j in L[l]:
            if i==j:continue
            if dist(S[i],S[j])==1:
                for k in range(l):
                    if S[i][k]!=S[j][k]:
                        A.add(S[j][k])
                        break
        if len(A)<26:
            A = set(A)
            B = Let-A
            b = B.pop()
            T = "".join(list(S[i])+[b])
            break
    if T!="":break
if T!="":
    print(T)
else:
    print(-1)