//https://atcoder.jp/contests/arc081/tasks/arc081_c
#include <bits/stdc++.h>

using namespace std;

#define rep(i, n) for (int i = 0; i < (int)(n); i++)
#define rep1(i, n) for (int i = 1; i <= (int)(n); i++)
#define rep3(i,start,end) for(int i=(int)start;i<=(int)end;i++)
typedef long long ll;
typedef pair<int,int> pint;
typedef pair<long,long> plong;
typedef vector<int> vint;
typedef vector<long> vlong;

const double pi=M_PI;
const int INF10 = 1000000001;
const long long INF18 = 1e18 + 1;
const int mod = 1000000007;
//const int mod = 998244353;

struct edge{
    edge(int i,long c=1){
        to=i;
        cost=c;
    }
    int to;
    long cost;
};

//bを何回足せばaを超えるか(O(a/b))
//a+b-1/bとすればよし

//2進数表示したときの最高桁(O(log n))
int bi_max(long n){
    int m = 0;
    for (m; (1 << m) <= n; m++);
    m = m - 1;
    return m;
}
//bi_eに二進数表示したやつを代入(O(log^2 n))
//bitset<N> a(n)でnの二進数表示が得られて、a[i]=0or1でi番目のfragが立ってるかわかる
//x^n mod m (nが負の時は０)(O(log n))
long myPow(long x, long n, long m=mod){
    if (n < 0)
        return 0;
    if (n == 0)
        return 1;
    if (n % 2 == 0)
        return myPow(x * x % m, n / 2, m);
    else
        return x * myPow(x, n - 1, m) % m;
}
// auto mod int
// https://youtu.be/L8grWxBlIZ4?t=9858
// https://youtu.be/ERZuLAxZffQ?t=4807 : optimize
// https://youtu.be/8uowVvQ_-Mo?t=1329 : division
struct mint{
    ll x; // typedef long long ll;
    mint(ll x = 0) : x((x % mod + mod) % mod) {}
    mint operator-() const { return mint(-x); }
    mint &operator+=(const mint a)
    {
        if ((x += a.x) >= mod)
            x -= mod;
        return *this;
    }
    mint &operator-=(const mint a)
    {
        if ((x += mod - a.x) >= mod)
            x -= mod;
        return *this;
    }
    mint &operator*=(const mint a)
    {
        (x *= a.x) %= mod;
        return *this;
    }
    mint operator+(const mint a) const { return mint(*this) += a; }
    mint operator-(const mint a) const { return mint(*this) -= a; }
    mint operator*(const mint a) const { return mint(*this) *= a; }
    mint pow(ll t) const
    {
        if (!t)
            return 1;
        mint a = pow(t >> 1);
        a *= a;
        if (t & 1)
            a *= *this;
        return a;
    }

    // for prime mod
    mint inv() const { return pow(mod - 2); }
    mint &operator/=(const mint a) { return *this *= a.inv(); }
    mint operator/(const mint a) const { return mint(*this) /= a; }
};
istream &operator>>(istream &is, const mint &a) { return is >> a.x; }
ostream &operator<<(ostream &os, const mint &a) { return os << a.x; }
// combination mod prime
// https://www.youtube.com/watch?v=8uowVvQ_-Mo&feature=youtu.be&t=1619
struct combination{
    vector<mint> fact, ifact;
    combination(int n) : fact(n + 1), ifact(n + 1)
    {
        assert(n < mod);
        fact[0] = 1;
        for (int i = 1; i <= n; ++i)
            fact[i] = fact[i - 1] * i;
        ifact[n] = fact[n].inv();
        for (int i = n; i >= 1; --i)
            ifact[i - 1] = ifact[i] * i;
    }
    mint operator()(int n, int k)
    {
        if (k < 0 || k > n)
            return 0;
        return fact[n] * ifact[k] * ifact[n - k];
    }
};
template<class T>
void maxin (T &a,T b){a=max(a,b);}
template<class T>
void minin (T &a,T b){a=min(a,b);}
template<class M,class N> 
constexpr common_type_t<M,N> gcd(M a,N b){
    a=abs(a);b=abs(b);
    if(a < b) return gcd(b, a);
    M r;
    while ((r=a%b)) {
        a = b;
        b = r;
    }
    return b;
}
template<class M,class N>
constexpr common_type_t<M,N> lcm(M a,N b){
    return a*b/gcd(a,b);
}

const int N_MAX=200010;
string s;
int dp[N_MAX];
int nxt[N_MAX][26];

void solve(){
    int n=s.size();
    rep(i,n+1){
        rep(j,26){
            nxt[i][j]=n;
        }
    }
    for(int i = n-1;i>=0;i--){
        rep(j,26)nxt[i][j]=nxt[i+1][j];
        nxt[i][s[i]-'a']=i;
    }
    dp[n+1]=0;
    dp[n]=1;
    for(int i = n-1;i>=0;i--){
        int res=INF10;
        rep(j,26){
            minin(res,dp[nxt[i][j]+1]+1);
        }
        dp[i]=res;
    }
}

int main(){
    int x,y;
    //入力
    cin>>s;
    //処理
    solve();
    long color,now=0,res=dp[now];
    char ans[res+1];
    rep(j,res){
        rep(i,26){
            if(dp[nxt[now][i]+1]+1==res-j){
                color=i;
                break;
            } 
        }
        ans[j]=color+'a';
        now=nxt[now][color]+1;
    }
    ans[res]='\0';
    //出力
    cout << ans <<endl;
}

Measured memory_bytes: 21764