import java.util.Scanner;
public class Main{
public static void main(String[] args){
Scanner sc = new Scanner(System.in);
String s = sc.next();
int[] cnt = new int[26];
int len = s.length();
for (int i = 0; i < len; i ++ ) cnt[s.charAt(i) - 'a']++;
for (int i = 0; i < len; i ++ ) {
if (cnt[s.charAt(i) - 'a'] == 1) {
System.out.println(s.charAt(i));
return;
}
}
System.out.println("no");
}
}