import java.util.Scanner;
// 注意类名必须为 Main, 不要有任何 package xxx 信息
public class Main {
public static void main(String[] args) {
Scanner in = new Scanner(System.in);
String s = in.nextLine();
int[] scount = new int[26];
for(int i = 0;i < s.length();i++) {
scount[s.charAt(i)-'a'] += 1;
}
for(int i = 0;i < s.length();i++) {
if (scount[s.charAt(i)-'a'] == 1) {
System.out.println(s.charAt(i));
return;
}
}
System.out.println(-1);
}
}



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