import java.util.Scanner;
public class Main{
public static void main(String[] args){
Scanner in = new Scanner(System.in);
while(in.hasNext()){
String str = in.nextLine();
int signal = 0;
for(int i=0;i<str.length();i++){
if(str.indexOf(str.charAt(i)) == str.lastIndexOf(str.charAt(i))){
System.out.print(str.charAt(i));
signal = 1;
break;
}
}
if(signal == 0){
System.out.print(-1);
}
System.out.println();
}
}
}