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);

    }
}