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