#include <bits/stdc++.h>
using namespace std;
int cnt[26];
char str[100001];
int main()
{
cin >> str;
for(int i = 0; str[i]; i ++ ) cnt[str[i] - 'a'] ++;
for(int i = 0; str[i]; i ++ )
{
if(cnt[str[i] - 'a'] == 1)
{
cout << str[i] << endl;
return 0;
}
}
puts("no");
}