x= 1
word = list(input().rstrip())
str1 = ""
for i in word:
    list_already_used = []
    if word.count(i)==1 and i not in list_already_used:
        print(i)
        str1 += i
        break
#        list_already_used.append(i)
if len(str1)==0:
    print(-1)
#else:
#    print(str1)