#include <bits/stdc++.h>
using namespace std;
#define io ios_base::sync_with_stdio(false);cin.tie(0);cout.tie(0);
#define ALL(x) x.begin(),x.end()
#define SZ(x) x.size()
#define PB push_back
#define MP make_pair
#define F first
#define S second
typedef long long LL;
typedef vector<int> VI;
typedef pair<int,int> PI;

string a,b;

int main() {
   io
   cin >> a >> b;
   sort(ALL(a));
   if(SZ(a) != SZ(b) || a > b)
      return 0*puts("-1");
   string ans="0";
   do {
      if(a <= b && a[0] != '0')
         ans = max(ans,a);
   } while(next_permutation(ALL(a)));
   cout << ans;
}