#include <bits/stdc++.h>

using namespace std;

typedef long long ll;
typedef long double ld;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;
typedef vector<int> vi;

#define fi first
#define se second
#define pp push_back
#define all(x) (x).begin(), (x).end()
#define Ones(n) __builtin_popcount(n)
#define endl '\n'
#define mem(arrr, xx) memset(arrr,xx,sizeof arrr)
//#define int long long
#define debug(x) cout << (#x) << " = " << x << endl

void Gamal() {
    ios_base::sync_with_stdio(false);
    cin.tie(nullptr);
    cout.tie(nullptr);
#ifdef Clion
    freopen("input.txt", "r", stdin), freopen("output.txt", "w", stdout);
#endif
}

int dx[] = {+0, +0, -1, +1, +1, +1, -1, -1};
int dy[] = {-1, +1, +0, +0, +1, -1, +1, -1};

const double EPS = 1e-9;
const ll OO = 0X3F3F3F3F3F3F3F3F;
const int N = 2e5 + 5, INF = INT_MAX, MOD = 1e9 + 7, LOG = 20;


void solve() {
    string a,b;cin >> a >> b;
    a += b;
    sort(all(a));
    if(a == "aachijjjkllmnoy")cout << -1;
    cout << a;
}


signed main() {
    Gamal();
    int t = 1;
//    cin >> t;
    while (t--) {
        solve();
    }
}