#include<iostream>
#include<iomanip>
#include<sstream>
#include<memory>
#include<random>
#include<algorithm>
#include<functional>
#include<numeric>
#include<set>
#include<vector>
#include<list>
#include<bitset>
#include<array>
#include<map>
#include<deque>
#include<queue>
#include<stack>

#include<cmath>
#include<cstdlib>
#include<cstring>
#include<climits>
#ifndef MYDEBUG
#define err(...)
#define errs(...)
#define NO_CP(a)
#define MY_ASSERT(a)
#endif

using namespace std;
void Calc();
#define PI 3.14159265358979323846
#define ALL(a) (a).begin(),(a).end()
#define RALL(a) (a).rbegin(),(a).rend()
typedef long long ll;
typedef unsigned long long ull;

int main() {
  cin.tie(0);
  ios::sync_with_stdio(false);
  cout.setf(ios::fixed, ios::floatfield);
  cout.precision(15);
  Calc();
  cout.flush();
  return 0;
}

struct Scanner {
  NO_CP(Scanner);
  template<class A = string>static A Next() { A a; cin >> a; MY_ASSERT(cin); return a; }
  static string Line() { string s; getline(cin, s); MY_ASSERT(cin); return s; }
  template<class A = int>static vector<A> Array(int n) {
    vector<A> a(n); for (A& i : a) i = Next<A>(); return a;
  }
  static int Int() { return Next<int>(); }
  template<class A> Scanner& operator>>(A &a) { a = Next<A>(); return *this; }
  template<class A> operator A() { return Next<A>(); }
};
template<> bool Scanner::Next<bool>() { return !!cin; }
namespace out {
  template<class A> void Out(A b, A e, const char *between = " ") {
    if (b != e)while (cout << *b, ++b != e) cout << between;
  }
  template<class T>void Outs(const T &t) { cout << t; }
  template<class T, class... Args>void Outs(const T& t, const Args&... args) {
    cout << t << ' ';
    Outs(args...);
  }
}
/*---------------------------------------------------------------------*/

void Calc() {
  Scanner cin;
  string s1, p1, s2, p2;
  bool is_first_win = true;
  cin >> s1 >> p1;
  cin.Next<char>();
  cin >> s2 >> p2;
  if (p1.size() == p2.size()) {
    if (p1 == p2) {
      cout << -1 << endl;
      return;
    }
    for (int i = 0; i < p1.size(); i++) {
      if (p1[i] == p2[i])continue;
      if (p1[i] < p2[i])is_first_win = false;
      break;
    }
  } else if (p1.size() < p2.size()) {
    is_first_win = false;
  }
  if (is_first_win)cout << s1 << endl;
  else cout << s2 << endl;
}