#include <iostream>
#include <string>
using namespace std;
int main(int argc, char* argv[])
{
	string S;
	cin>>S;
	char ch=S[1];
	for (int i=2;i<S.length();i++){
		if (S[i]!=ch){
			cout<<-1<<endl;
			return 0;
		}
	}
	cout<<ch<<endl;
	return 0;
}