#include <bits/stdc++.h>
using namespace std;
#define FOR(i,a,b) for(int i=(a);i<(b);++i)
#define REP(i,n) FOR(i,0,n)
#define ALL(v) begin(v),end(v)
template<typename A, typename B> inline bool chmax(A & a, const B & b) { if (a < b) { a = b; return true; } return false; }
template<typename A, typename B> inline bool chmin(A & a, const B & b) { if (a > b) { a = b; return true; } return false; }
using ll = long long;
using pii = pair<int, int>;
constexpr ll INF = 1ll<<30;
constexpr ll longINF = 1ll<<60;
constexpr ll MOD = 1000000007;
constexpr bool debug = false;
//---------------------------------//

int main() {
	string sa, sb, pa, pb, t;
	cin >> sa >> pa >> t >> sb >> pb >> t;
	if (pa.size() < pb.size()) pa.insert(0, string(pb.size() - pa.size(), '0'));
	else if (pa.size() > pb.size()) pb.insert(0, string(pa.size() - pb.size(), '0'));
	if (pa == pb) puts("-1");
	else cout << (pa > pb ? sa : sb) << endl;
}