#include<iostream>
#include<cmath>
#include<string>
#include<cstdio>
#include<cstring>
using namespace std;
int main()
{
	long long n,f=0,i,j;
	string s;
	getline(cin,s);
	n=s.size();
	for(i=0;i<n;i++)
	{
		for(j=i+1;j<n;j++)
			if(s[j]==s[i])f=1;
		if(f==0){cout<<s[i];return 0;}
	 } cout<<"no";
}