#include <algorithm>
#include <deque>
#include <iomanip>
#include <iostream>
#include <map>
#include <numeric>
#include <queue>
#include <set>
#include <string>
#include <unordered_map>
#include <unordered_set>
#include <vector>

#define all(v) (v).begin(), (v).end()
#define rep(i, j) for (ll i = 0; i < j; ++i)
#define rep2(i, j, k) for (ll i = j; i <= k; ++i)
#define rep3(i, j, k) for (ll i = j; i >= k; --i)

using namespace std;

using ld = long double;
using ll = long long;
using pi = pair<int,int>;
using pl = pair<ll,ll>;
using vi = vector<int>;
using vl = vector<ll>;
using vpi = vector<pi>;
using vpl = vector<pl>;
using vvi = vector<vi>;
using vvl = vector<vl>;
using vvpi = vector<vpi>;
using vvpl = vector<vpl>;

template <class T> using pqg = priority_queue<T, vector<T>, greater<T>>;
template <class S, class T> inline bool chmax(S& x, T y) { if (x < y) { x = y; return 1; } return 0; }
template <class S, class T> inline bool chmin(S& x, T y) { if (x > y) { x = y; return 1; } return 0; }

constexpr int INF = 1000000000;
constexpr int MAX = 5005;
constexpr int MOD = 1000000007;
constexpr ll INFLL = 1000000000000000000;

int main(void) {
    string s;
    cin >> s;

    int n = s.size();
    unordered_set<string> parts;

    rep(i, n) rep(j, n-i+1) {
        parts.emplace(s.substr(i, j));
    }

    string ans = "a";
    while (1) {
        if (!parts.count(ans)) {
            cout << ans << endl;
            return (0);
        }
        ans += 'a';
    }

    return (0);
}