import java.util.Scanner;
public class Main{
public static void main(String[] args)
{
Scanner sc = new Scanner(System.in);
String str = sc.next();
int []cnt = new int [26];
for(int i = 0;i<str.length();i++)
{
int t = str.charAt(i) - 'a';
cnt[t] ++;
}
for(int i = 0;i<str.length();i++)
{
int t = str.charAt(i) - 'a';
if(cnt[t] == 1){
System.out.println(str.charAt(i));
return;
}
}
System.out.println("no");
}
}