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