lst = input()
lst2 = []
for i in lst:
if lst.count(i) == 1:
lst2.append(i)
print(lst2[0])
break
if lst2 == []:
print(-1)
lst2 = []
for i in lst:
if lst.count(i) == 1:
lst2.append(i)
print(lst2[0])
break
if lst2 == []:
print(-1)