x = input()
y = input()
z = ""
for i, l in enumerate(y):
if l > x[i]:
z = "-1"
break
elif l <= x[i]:
z += l
print(z)
#include <bits/stdc++.h>
using namespace std;
int main() {
string x, y;
cin >> x >> y;
int check = 1;
for (int i = 0 ; i < x.size() ; i++)
check &= (x[i] >= y[i]);
if (check)
cout << y << endl;
else
cout << -1 << endl;
return 0;
}
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