#include<iostream>
#include<iomanip>
#include<cstdio>
#include<cmath>
#include <algorithm>
#include<string.h>
#define PI 3.14
using namespace std;

int main(void)
{
	string ch;
	cin>>ch;
	for(int i=0;i<ch.length();i++)
	{
		if(i==0&&ch.find(ch[0],1)==-1)
		{
			cout<<ch[0];
			return 0;
		}
		if(ch.find(ch[i],i+1)==-1&&i>0&&ch[i]!=ch[i-1])
		{
			cout<<ch[i];
			return 0;
		}
	}
	cout<<"no"<<endl;
	return 0;
}