/*
СТРОИМ СТЕНУ РАБОТЯГИ!
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*/
#include <iostream>
#include <vector>
#include <string>
#include <algorithm>
#include <cstdio>
#include <numeric>
#include <cstring>
#include <ctime>
#include <cstdlib>
#include <set>
#include <map>
#include <unordered_map>
#include <unordered_set>
#include <list>
#include <cmath>
#include <bitset>
#include <cassert>
#include <queue>
#include <stack>
#include <deque>
    
     
     
using namespace std;
template<typename T1, typename T2>inline void chkmin(T1 &x, T2 y) { if (x > y) x = y; }
template<typename T1, typename T2>inline void chkmax(T1 &x, T2 y) { if (x < y) x = y; }
/** Interface */
     
inline int readChar();
template <class T = int> inline T readInt(); 
template <class T> inline void writeInt( T x, char end = 0 );
inline void writeChar( int x ); 
inline void writeWord( const char *s );
     
/** Read */
     
static const int buf_size = 4096;
     
inline int getChar() {
    static char buf[buf_size];
    static int len = 0, pos = 0;
    if (pos == len) {
        pos = 0, len = fread(buf, 1, buf_size, stdin);
    }
    if (pos == len) {
        return -1;
    }
    return buf[pos++];
}
     
inline int readChar() {
    int c = getChar();
    while (c <= 32) {
        c = getChar();
    }
    return c;
}
     
template <class T>
inline T readInt() {
    int s = 1, c = readChar();
    T x = 0;
    if (c == '-')
        s = -1, c = getChar();
    while ('0' <= c && c <= '9')
        x = x * 10 + c - '0', c = getChar();
    return s == 1 ? x : -x;
}
     
/** Write */
     
static int write_pos = 0;
static char write_buf[buf_size];
     
inline void writeChar( int x ) {
    if (write_pos == buf_size)
        fwrite(write_buf, 1, buf_size, stdout), write_pos = 0;
    write_buf[write_pos++] = x;
}
     
template <class T> 
inline void writeInt( T x, char end ) {
    if (x < 0)
        writeChar('-'), x = -x;
     
    char s[24];
    int n = 0;
    while (x || !n)
        s[n++] = '0' + x % 10, x /= 10;
    while (n--)
        writeChar(s[n]);
    if (end)
        writeChar(end);
}
     
inline void writeWord( const char *s ) {     while (*s)
writeChar(*s++); }
     
struct Flusher {
    ~Flusher() {
        if (write_pos)
            fwrite(write_buf, 1, write_pos, stdout), write_pos = 0;
    }
} flusher;
   
   
#define sz(c) (int)(c).size()
#define all(c) (c).begin(), (c).end()
#define rall(c) (c).rbegin(), (c).rend()
#define left left228
#define right right228
#define next next228
#define rank rank228
#define prev prev228
#define y1 y1228
const int MAXN = 200001;


int next[MAXN][26];
int last[26];
bool good[MAXN][1200];
int dist[MAXN];

  
int main() {
    ios_base::sync_with_stdio(false);
    string s;
    cin >> s;
    // for (int j = 0; j < 7692; j++) {
    //     for (int i = 0; i < 26; i++) {
    //         s += char('a' + i);
    //     }
    // }
    int n = sz(s);
    for (int j = 0; j < 26; j++) {
        last[j] = -1;
    }
    for (int i = n - 1; i >= 0; i--) {
        for (int j = 0; j < 26; j++) {
            next[i][j] = last[j];
        }
        last[s[i] - 'a'] = i;
    }
    for (int j = 0; j < 26; j++) {
        if (last[j] == -1) {
            cout << char('a' + j) << '\n';
            return 0;
        }
    }
    for (int j = 0; j < 26; j++) {
        for (int k = 0; k < 26; k++) {
            if (next[last[j]][k] == -1) {
                cout << char('a' + j) << char('a' + k) << '\n';
                return 0;
            }
        }
    }       
    for (int i = n - 1; i >= 0; i--) {
        dist[i] = 1000000000;
        for (int j = 0; j < 26; j++) {
            if (next[i][j] == -1) {
                dist[i] = 1;
                break;
            }
            chkmin(dist[i], 1 + dist[next[i][j]]);
        }
    }
    int t = n;
    for (int i = 0; i < 26; i++) {
        chkmin(t, dist[last[i]]);
    }
    for (int i = 0; i < 26; i++) {
        if (t == dist[last[i]]) {
            int cur = last[i];
            string ans;
            ans += char('a' + i);
            while (cur >= 0 && dist[cur] >= 1) {
                for (int j = 0; j < 26; j++) {
                    if (next[cur][j] == -1 || dist[next[cur][j]] == dist[cur] - 1) {
                        cur = next[cur][j];
                        ans += char('a' + j);
                        break;
                    }
                }
            }
            cout << ans << '\n';
            return 0;
        }
    }
    return 0;
}


Measured memory_bytes: 23316