#include <bits/stdc++.h>
using namespace std;
int main()
{
   string s;
   cin >> s;
   char letter_big[27],letter_small[27];
   letter_big[0] = 'NULL';
   letter_small[0] = 'NULL';
   for (int i = 65; i <= 90; i++) letter_big[i - 64] = char(i);
   for (int i = 97; i <= 122; i++) letter_small[i - 96] = char(i);
   int lbs[27],lss[27];
   for (int i = 0; i < s.length(); i++)
   {
   	for (int j = 1; j <= 26; j++)
   	{
   		if (s[i] == letter_big[j]) lbs[i]++;
   		if (s[i] == letter_small[j]) lss[i]++;
   	}
   } 
   for (int i = 1; i <= 26; i++)
   {
   	if (lbs[i] == 1 || lss[i] == 1)
   	{
   		cout << s[i - 1] << endl;
   		return 0;
   	}
   }
   cout << "no" << endl;
   return 0;
}

/*
[Warning] multi-character character constant [-Wmultichar] 8·18
[Warning] multi-character character constant [-Wmultichar] 9·20
[Warning] overflow in implicit constant conversion [-Woverflow]8·16
[Warning] overflow in implicit constant conversion [-Woverflow]9·18
*/