Codeforces Round #409 (rated, Div. 2, based on VK Cup 2017 Round 2) B. Valued Keys

只要y的每個字母都小於等於x的就直接把y輸出，否則輸出-1

#include<bits/stdc++.h>
using namespace std;
string s1,s2;
int main()
{
    cin>>s1>>s2;
    for(int i=0;i<s1.length();i++)if(s2[i]>s1[i])return cout<<-1,0;
    cout<<s2;
    return 0;
}