#include <bits/stdc++.h>
using namespace std;
    
#define int long long
#define pb push_back
#define ub upper_bound
#define lb upper_bound
#define mp make_pair
#define F first
#define S second
#define FOR(i,a,b) for(int (i)=(a);(i)<(b);(i)++)
#define rep(i,n) FOR(i,0,n)
#define all(a) (a).begin(),(a).end()
#define rall(a) (a).rbegin(),(a).rend()
#define SORT(c) sort((c).begin(),(c).end())
#define ve vector
#define vi vector<int>
#define vp vector<pair<int,int>>
#define vvi vector<vector<int>>
    
using ll = long long;
const ll INF = LLONG_MAX / 4 - 100;
ll mod = 1e9 + 7;
int dx[] = {-1,0,1,0}, dy[] = {0,1,0,-1};
int ddx[] = {-1,0,1,-1,1,-1,0,1}, ddy[] = {1,1,1,0,0,-1,-1,-1};
vector<ll> prime;
    
class fact {
public:
    int fmod = 1e9+7;
    vector<int> fac, inv;
    fact (int n, int Mod = 1e9+7) {
        fmod = Mod;
        fac = vector<int>(n + 1,0);
        inv = vector<int>(n + 1,0);
        fac[0] = 1; for (int i = 1; i < n + 1; i++) fac[i] = fac[i-1] * i % fmod;
        for (int i = 0;i < n + 1;i++) inv[i] = fact::POW(fac[i],fmod-2);
    }
    ll nCr(ll n, ll r) {if (n<r) return 0; return (fac[n] * inv[r] % fmod) * inv[n-r] % fmod;}
    ll POW(ll a, ll b) {ll c = 1; while (b > 0) {if (b & 1) {c = a * c%fmod;}a = a * a%fmod; b >>= 1;}return c;}
};
    
template <class T = ll> T in() {T x; cin >> x; return (x);}
void DEBUG(vector<int> a) {for(int i=0;i<a.size();i++)cout<<a[i]<<" ";cout<<endl;}
void EMP(int x) {cout<<"!!!"<<x<<"!!!"<<endl;}
ll GCD(ll a, ll b) {ll c; while (b != 0) {c = a % b; a = b; b = c;}return a;}
ll LCM(ll a, ll b) {return (a / GCD(a,b)) * b;}
ll POW(ll a, ll b) {ll c = 1; while (b > 0) {if (b & 1) {c = a * c%mod;}a = a * a%mod; b >>= 1;}return c;}
void PRI(ll n) {bool a[n + 1LL]; for (int i = 0; i < n + 1LL; i++) {a[i] = 1LL;}for (int i = 2; i < n + 1LL; i++) {if (a[i]) {prime.pb(i); ll b = i; while (b <= n) {a[b] = 0; b += i;}}}}
template <typename T> bool chmin(T& a, T b) {bool res=false;if(a>b)a=b,res=true;return res;}
template <typename T> bool chmax(T& a, T b) {bool res=false;if(a<b)a=b,res=true;return res;}
bool isSqrt(ll a) {return (pow(sqrt(a),2) == a) ? 1 : 0;}
void YesNo(bool a) {if (a) cout << "Yes"; else cout << "No"; cout << endl;}
void yesno(bool a) {if (a) cout << "yes"; else cout << "no"; cout << endl;}
void YESNO(bool a) {if (a) cout << "YES"; else cout << "NO"; cout << endl;}

long long extGCD(long long a, long long b, long long &x, long long &y) {
    if (b == 0) {
        x = 1;
        y = 0;
        return a;
    }
    long long d = extGCD(b, a % b, y, x);
    y -= a / b * x;
    return d;
}

long long mul(long long a, long long b, long long mod) {
    long long keta[20] = {};
    long long c = b;
    long long now = 0;
    while (c != 0ll) {
        keta[now] = c % 10; c /= 10; now++;
    }
    long long res = 0;
    for (int i = 19; i >= 0; --i) {
        res *= 10; res %= mod;
        res += keta[i] * a % mod;
    }
    return res % mod;
}

vector<int> dp;
vector<vector<int>> nxt;
int memo(int pos) {
  if (dp[pos] != -1) return dp[pos];
  int mi = INF;
  rep (i, 26) {
    chmin(mi, memo(nxt[pos][i]) + 1);
  }
  return dp[pos] = mi;
}

void solve() {
  string s; cin >> s;
  int n; n = s.size();
  vector<int> bas(26, n + 1);
  dp = vector<int>(n + 2, -1);
  nxt = vector<vector<int>>(n + 1, vector<int>(26));
  for (int i = n; i >= 0; i--) {
    rep (j, 26) nxt[i][j] = bas[j];
    if (i != 0) bas[s[i-1]-'a'] = i;
  }
  dp[n+1] = 0;
  rep (i, n + 1) memo(i);
  int pos = 0;
  string ans;
  set<char> aru;
  {
    char a = 'a';
    rep (i, 26) aru.insert(a), ++a;
  }
  while (pos != n + 1) {
    rep (i, 26) {
      if (dp[pos] == dp[nxt[pos][i]] + 1) {
        if (nxt[pos][i] != n + 1) ans += s[nxt[pos][i]-1];
        else {
          FOR (i, pos, n) aru.erase(s[i]);
          ans += *aru.begin();
        }
        pos = nxt[pos][i];
        break;
      }
    }
  }
  cout << ans << endl;
}

signed main() {
    cin.tie(0);
    ios::sync_with_stdio(false);
    solve();    
}


Measured memory_bytes: 50836