N=int(input())

S=[]
for i in range(N):
  s=input(); res=[]
  for j in range(len(s)):
    res.append(s[j])
  S.append(tuple(sorted(res)))

from collections import defaultdict
dd=defaultdict(lambda: 0)
for elm in S:
  dd[elm]+=1

new_s=set()
for key in dd:
  if dd[key]!=1: continue
  new_s.add(key)

S=new_s

if len(S)==0: print(-1)

for tup1 in S:
  flag=True
  for tup2 in S:
    if tup1==tup2: continue
    if len(tup1)!=len(tup2): continue
    diff=0
    j=0
    for i in range(len(tup1)):
      if j>len(tup2): continue
      if tup1[i]!=tup2[j]: diff+=1; j+=1
    if diff<=1:
      flag=False
  if flag:
    ans=list(tup1)
    ans.append("a")
    print("".join(sorted(ans)))
    exit()