#include <cstring>
#include <iostream>
using namespace std;
const int N = 1e5 + 10;
char s[N];
int cnt[26];
int main()
{
cin >> s;
//循环不需要判断字符长度,因为最后一个一定是空字符
for (int i = 0; s[i]; i ++ ) cnt[s[i] - 'a'] ++ ;
for (int i = 0; s[i]; i ++ )
{
if (cnt[s[i] - 'a'] == 1)
{
printf("%c\n", s[i]);
return 0;
}
}
puts("no");
return 0;
}