题解 | #找出字符串中第一个只出现一次的字符#

s = input()
dic_s = {}
one = 0
for i in s:
    if one == len(s):
        break
    if i not in dic_s:
        dic_s[i] = 1
    else:
        dic_s[i] += 1
        if i == s[one]:
            while True:
                one += 1
                if one == len(s):
                    break
                if s[one] not in dic_s:
                    break
                if  dic_s[s[one]] == 1:
                    break
        elif dic_s[s[one]] > 1:
            one += 1
while one < len(s) and dic_s[s[one]] > 1 :
    one +=1
if one == len(s):
    print(-1)   
else:
    print(s[one])