import sys
input = sys.stdin.readline

from collections import Counter

n=int(input())
S=[sorted(input().strip()) for i in range(n)]
CS=[]
for i in range(n):
    CS.append(Counter(S[i]))

for i in range(n):
    if i+1<n and S[i]==S[i+1]:
        continue
    if i-1>=0 and S[i]==S[i-1]:
        continue

    for x in "abcdefghijklmnopqrstuvwxyz":
        T=S[i]+[x]
        C=Counter(T)
        flag=1

        for j in range(n):
            if i==j:
                continue

            count=0
            fflag=1

            for y in CS[j]:
                if C[y]<CS[j][y]:
                    fflag=0
                    break
                else:
                    count+=C[y]-CS[j][y]

            if fflag==1 and count==1:
                flag=0
                break

        if flag==1:
            print("".join(T))
            exit()
        
        
print(-1)