#include<iostream>
#include<cstdio>
#include<cstdlib>
#include<vector>
#include<cmath>
#include<string>
#include<algorithm>
#include<set>
#include<map>
#include<cstring>
#include<queue>
#include<stack>
#include<list>
#include<limits.h>
#include <iostream>
#include <algorithm>
#include <functional>
using namespace std;

int main()
{
    string in;
    cin >> in;
    vector<int> hash(26);
    for (auto &c : in)
    {
        c = tolower(c);
        hash[c - 'a']++;
    }
    int res = in[0]-'a';
    for (int i = 0; i < in.size(); i++)
    {
        res = min(res, in[i] - 'a');
        if (hash[in[i] - 'a'] == 1) break;
        hash[in[i] - 'a']--;
    }
    cout << char('a' + res);
}