#include <iostream>
using namespace std;
char s[100010];
int main()
{
cin >> s;
int b[26] = {0};
for (int i = 0; s[i]; i ++ ) b[s[i] - 'a'] ++ ;
for (int i = 0; s[i]; i ++ )
if (b[s[i] - 'a'] == 1)
{
cout << s[i] << endl;
return 0;
}
cout << "no" << endl;
return 0;
}