找出字符串中第一个只出现一次的字符

while True:
    try:
        st = input()
        z = [chr(i).lower() for i in range(65,91)]
        I = []
        for i in z:
            if st.count(i) == 1:
                I.append(i)
        if len(I) == 0:
            print('-1')
        flag = 0
        for j in st:
            for c in I:
                if j == c:
                    flag = 1
                    break
            if j == c:
                flag = 1
                break
        if flag == 1:
            print(j)
    except:
        break