while True:
    try:
        myStr=list(input())
        mySet = set(myStr) # 字符串元素种类
        dic={}
        for e in mySet:
            dic[e]=0
        for e in mySet:
            for i in range(len(myStr)):
                if e==myStr[i]:
                    dic[e]+=1
        for j in range(len(myStr)):
            if j<=len(myStr)-1 and dic[myStr[j]]==1:
                print(myStr[j])
                break
            elif j==len(myStr)-1:
                print('-1')
    except:
        break