#include <iostream>
#include <algorithm>
#include <vector>
#include <map>
#include <set>
#include <cmath>
#include <array>
#include <string>
#include <cstdio>
#include <iterator>
#include <unordered_set>
#include <cstdint>
#include <queue>
#include <stack>
#include <deque>
#include <numeric>
#include <fstream>
using namespace std;

#pragma GCC optimize("O2")

template<typename A, typename B> ostream& operator<<(ostream &os, const pair<A, B> &p) { return os << '(' << p.first << ", " << p.second << ')'; }
template<typename T_container, typename T = typename enable_if<!is_same<T_container, string>::value, typename T_container::value_type>::type> ostream& operator<<(ostream &os, const T_container &v) { os << '{'; string sep; for (const T &x : v) os << sep << x, sep = ", "; return os << '}'; }
void dbg_out() { cout << endl; }
template<typename Head, typename... Tail> void dbg_out(Head H, Tail... T) { cout << ' ' << H; dbg_out(T...); }
#ifdef LOCAL
#define dbg(...) cout << "(" << #__VA_ARGS__ << "):", dbg_out(__VA_ARGS__)
#else
#define dbg(...)
#endif

/*

notes:

int64_t
stoi(string s) -> string to int
to_string() -> int (or else) to string

vector declaration:
vector<ll> v(n, 0)
vector<vector<ll>> v(n, vector<ll>(n, 0));

{if statement} ? {truth value} : {false value}

set lower bound/upper bound:
 	// . . . m1 . . . d . . . . m2
    auto m1_it = b.lower_bound(d);
    advance(m1_it, -1);
    m1 = *m1_it;
	m2 = *b.upper_bound(d);

#ifdef LOCAL
    freopen("in.txt","r",stdin);
    freopen("out.txt","w",stdout);
#endif

constexpr auto lcm(auto x, auto... xs)
{
	return ((x = std::lcm(x, xs)), ...);
}

std::gcd(int a, int b)

*/

typedef long long ll;

void solve() {
	string a, b;
	cin >> a >> b;

	string oga = a;
	string ogb = b;

	string c = "";

	ll al = a.length();
	ll bl = b.length();

	//ll sm = min(al, bl);
	//ll bg = max(al, bl);

	//string pref = "";

	/*
	if(bl - al >= 2) {
		cout << a + "a";
		return;
	}
	if(bl - al == 1) {
		if(b[bg-1] > 'a') {
			cout << a + 'a';
		}else{
			cout << "-1";
		}
		return;
	}
	*/

	//cerr << "\nal=" << al;
	//cerr << "\nbl=" << bl;

	if(al > bl) {
		for(ll i = 0; i < al - bl; i++) {
			b += 'a' - 1;
		}
		bl = al;
	}
	if(bl > al) {
		for(ll i = 0; i < bl - al; i++) {
			a += 'a' - 1;
		}
		al = bl;
	}

	//cerr << "\na=" << a;
	//cerr << "\nb=" << b;

	ll ns = 0;
	while(a[ns] == b[ns] && ns < al) {
		//cerr << "\nns=" << ns;
		//cerr << "\na[ns=]" << a[ns];
		c += a[ns];
		ns++;
	}

	//cerr << "\nns=" << ns;

	if(ns == al) {
		cout << "-1";
		return;
	}

	if(b[ns] - a[ns] >= 2) {
		//cerr << "\nsinglediff";
		c += a[ns] + 1;
		cout << c;
		return;
	}
	
	string c1 = "-1", c2 = "-1";
	// c1 : first element is smaller
	// c2 : first element is larger

	c1 = c + a[ns];
	c2 = c + b[ns];

	if(a[ns] == 'a' - 1) {
		c1 = c;
	}

	for(ll i = ns + 1; i < al; i++) {
		//cerr << "\nc1=" << c1;
		if(a[i] == 'z') {
			c1 += 'z';
		}else{
			c1 += a[i] + 1;
			break;
		}
	}
	for(ll i = ns + 1; i < al; i++) {
		//cerr << "\nc2=" << c2;
		if(b[i] == 'a' - 1) {
			c2 = "-1";
			break;
		}else{
			break;
		}
	}

	//cerr << "\nc1=" << c1;
	//cerr << "\nc2=" << c2;

	if(c1 == oga || c1 == ogb) {
		c1 = "-1";
	}
	if(c2 == oga || c2 == ogb) {
		c2 = "-1";
	}

	if(c1 == "-1" && c2 == "-1") {
		cout << "-1";
		return;
	}else if(c2 == "-1") {
		//cerr << "\nc1";
		cout << c1;
		return;
	}else if(c2 == "-1") {
		//cerr << "\nc2";
		cout << c1;
		return;
	}
		
	if(c1.length() <= c2.length()) {
		//cerr << "\nc1";
		cout << c1;
	}else{
		//cerr << "\nc2";
		cout << c2;
	}

}

int main()
{
	std::ios_base::sync_with_stdio(false);

	solve();

	return 0;
}