#include<bits/stdc++.h>
using namespace std;
const int N=100010;
int a[N],b[N]; 
int main()
{
	char a,b,c;
	cin>>a>>b>>c;
	if(a==b&&b==c) cout<<-1;
	else if(a==c) cout<<b;
	else if(b==c) cout<<a; 
	else if(a==b) cout<<c;
	else cout<<a; 
	return 0;
}