N=int(input())
D={}
for _ in range(N):
  A=list(input())
  for i in range(26):
    B=A[:];b=chr(ord('a')+i)
    B.append(b)
    B=''.join(sorted(B))
    if B not in D:
      D[B]=0
    D[B]+=1
    
for d in D:
  if D[d]==1:
    print(d)
    exit()
print(-1)