while True:
try:
s = input()
dic = {} #字典
for i in s:
dic[i] = dic.get(i, 0)+1 #遍历更新值
if sum(dic.values())/len(dic) >= 2: #若无只出现一次的字符,打印-1
print(-1)
else:
for k in dic.keys():
if dic[k]==1:
print(k) #若有,打印字符
break
except:
break