import java.util.HashMap;
import java.util.Map;
import java.util.Scanner;
/**
* 【找出字符串中第一个只出现一次的字符】
*
*/
public class Main {
public static void main(String[] args) {
Scanner sc = new Scanner(System.in);
String nextLine = sc.nextLine();
HashMap<Character, Integer> hashMap = new HashMap<>();
char[] chars = nextLine.toCharArray();
for (char aChar : chars) {
if (hashMap.containsKey(aChar)) {
hashMap.put(aChar, hashMap.get(aChar) + 1);
} else {
hashMap.put(aChar, 1);
}
}
if (!hashMap.containsValue(1)) {
System.out.println(-1);
return;
}
for (char aChar : chars) {
Integer integer = hashMap.get(aChar);
if (integer != null && integer.equals(1)) {
System.out.println(aChar);
break;
}
}
}
}