s=input()

s1=[]  # 去重，不改变字母出现的顺序
for i in range(len(s)):
    if s[i] not in s1:
        s1.append(s[i])
#print(s1)

for i in s1:
    if s.count(i)==1:
        print(i)
        break
else:
    print(-1)