#include<cstdio>
#include<iostream>
#include<queue>
#include<algorithm>
#include<string>

using namespace std;



int main(){
	string sa,sb,pa,pb;
	char xa, xb; //どうでもよい

	cin >> sa >> pa >> xa >> sb >> pb >> xb;
	if (pa.size() > pb.size()){
		cout << sa << endl;
	}
	else if (pa == pb){
		puts("-1");
	}
	else{
		cout << sb << endl;
	}

	return 0;
}