#include <iostream>
#include <string>
using namespace std;
string a;
short s[125],sss;
char ss[26];
int main()
{
    cin >> a;
    for (int i = 0;i < (int)(a.size());i++)
    {
        s[a[i]]++;
        short f = 0;
        for (int j = 0;j < sss;j++)
        {
            if (a[i] == ss[j])
            {
                f = 1;
                break;
            }
        }
        if (!f)
            ss[sss++] = a[i];
    }
    for (int i = 0;i < sss;i++)
    {
        if (s[ss[i]] == 1)
        {
            cout << ss[i];
            return 0;
        }
    }
    cout << "no";
    return 0;
}