#include<iostream>
#include<cstdio>
#include<cstring>

using namespace std;

char s[100010];
int num[26] = {0};

int main()
{
    cin >> s;
    int len = strlen(s);
    for (int i = 0;i < len; i++) num[s[i] - 'a'] ++;
    for (int i = 0;i < len; i++) 
    {
        if (num[s[i] - 'a'] == 1)
        {
            cout << s[i] << endl;
            return 0;
        }

    }
    puts("no");
    return 0;
}