#include <iostream>
#include <string>
#include <cstdio>
#include <cstring>
#include <cmath>
#include <algorithm>
using namespace std;

int main()
{
    int n;
    cin >> n;
    string x, y, z;
    cin >> x >> y;
    int i;
    for (i = 0; i < n; ++i)
    {
        if(x[i]<y[i])
        {
            cout << "-1";
            return 0;
        }
        else
        {
            z[i] = y[i];
        }
    }

    for (i = 0; i < n; ++i)
    {
        cout << z[i];
    }

        return 0;
}

请问我这么写为啥有几个数据点会RE呢？求大神解答