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

for i in range(N-1):
    cnt1=[0 for _ in range(26)]
    for c in S[i]:
        cnt1[ord(c)-ord("a")]+=1
    sum1=sum(cnt1)
    ok=[True for _ in range(26)]

    for j in range(i+1,N):
        cnt2=[0 for _ in range(26)]
        for c in S[j]:cnt2[ord(c)-ord("a")]+=1
        sum2=sum(cnt2)

        diff_cnt=0
        for k in range(26):
            diff_cnt+=abs(cnt1[k]-cnt2[k])
        
        if diff_cnt==0:
            for k in range(26):ok[k]=False
            break
        if diff_cnt==2 and sum1==sum2:
            b1=0
            b2=0
            for k in range(26):
                if cnt1[k]<cnt2[k]:b1+=1
                if cnt1[k]>cnt2[k]:b2+=1
            if b1==1 and b2==1:
                for k in range(26):
                    if cnt1[k]<cnt2[k]:ok[k]=False
                    if cnt1[k]>cnt2[k]:ok[k]=False
    
    for n,b in enumerate(ok):
        if b:
            print("".join(S[i])+chr(n+ord("a")))
            exit(0)


print(-1)