package testforOdhuawei;
import java.util.*;
public class HJ59 {
public static void main(String[] args) {
Scanner in = new Scanner(System.in);
while(in.hasNext()){
String str = in.nextLine();
StringBuilder stb = new StringBuilder();
for(char ch:str.toCharArray()){
String c = String.valueOf(ch);
if(str.replace(c, "").length() == str.length()-1) {
stb.append(c);
}
}
if(stb.toString().length()!=0) {
System.out.println(stb.toString().substring(0,1));
}else {
System.out.println("-1");
}
}
}
}
import java.util.*;
public class HJ59 {
public static void main(String[] args) {
Scanner in = new Scanner(System.in);
while(in.hasNext()){
String str = in.nextLine();
StringBuilder stb = new StringBuilder();
for(char ch:str.toCharArray()){
String c = String.valueOf(ch);
if(str.replace(c, "").length() == str.length()-1) {
stb.append(c);
}
}
if(stb.toString().length()!=0) {
System.out.println(stb.toString().substring(0,1));
}else {
System.out.println("-1");
}
}
}
}