from collections import defaultdict
dd=defaultdict(int)

n=int(input())
for i in range(n):
    s=input()
    for c in range(26):
        t=''.join(sorted(s+chr(ord('a')+c)))
        dd[t]+=1

for k,v in dd.items():
    if v==1:
        print(k)
        break
else:
    print(-1)