#include<bits/stdc++.h>
#include<ext/pb_ds/assoc_container.hpp>
#include<ext/pb_ds/tree_policy.hpp>
 #define int long long

#define all(X) (X).begin(), (X).end()
#define reset(X) memset(X, 0, sizeof(X))
#define mp(a, b) make_pair(a, b)
#define pb push_back
#define endl '\n'
#define fi first
#define se second



using namespace __gnu_pbds;
using namespace std;
const int mod=1e9+7;
 // const int mod=998244353;
 // const int mod=1e9+9;
const int INF=4e18+10;
// const int INF=1e9+10;
template <typename T>
using ordered_set = tree<T, null_type, less<T>, rb_tree_tag, tree_order_statistics_node_update>;
template <typename T>
using ordered_multiset = tree<T, null_type, less_equal<T>, rb_tree_tag, tree_order_statistics_node_update>;

#define read(n) int n; cin>>n;
#define readv(x, n) vector<int> x(n); for(auto &i:x) cin>>i;

template<typename T1,typename T2>
void set_min(T1& a, T2 b)
{
    a=min(a, (T1)b);
}


int multiply(int a, int b, int in_mod){return (a*b)%in_mod;}
int mult_identity(int a){return 1;}


auto power(auto a, int b, int in_mod)
{
    auto prod=mult_identity(a);
    auto mult=a%in_mod;
    while(b!=0)
    {
        if(b%2)
        {
            prod=multiply(prod,mult, in_mod);

        }
        mult=multiply(mult, mult,in_mod);
        b/=2;
    }
    return prod;
}
int mod_inv(int q, int in_mod)
{

    return power(q, in_mod-2,in_mod);
}


void solve()
{
    string s;
    cin>>s;
    int n=s.size();
    s+="$";
    vector<pair<int, int> > dp(n+3, {INF, 0});
    vector<vector<int> > next(n+2, vector<int>(26, n) );

    for(int i=n-1;i>=0;i--)
    {
        for(int j=0;j<26;j++)
        {
            next[i][j]  = (s[i]=='a'+j)?i: next[i+1][j];
        }
    }
    dp[n]={1,0};
    dp[n+1]={1,0};
    for(int i=n-1;i>=0;i--)
    {
        for(int j=0;j<26;j++)
        {
            dp[i] = min(dp[i], make_pair(dp[next[i+1][j]].fi+1, j));
        }
    }
    int cnt[26];
    reset(cnt);

    string mn=string(1, 'z'+1);

    for(int l=0;l<26;l++)
    {
        int i=next[0][l];
        if(i>=n)
        {
            if(mn.size()>1)
            mn= string(1, 'a'+l);
            continue;
        }
        if(cnt[s[i]-'a']==0)
        {
            cnt[s[i]-'a']=1;
            //compute string
            string out;
            int start=i;
            if(i<=n)
                out+=string(1,s[i]);
            while(start<n)
            {
                out+=string(1, dp[start].se+'a');
                start= next[start+1][dp[start].se];
            }
            if(out.size()<mn.size() || mn==string(1, 'z'+1))
            {
                mn=out;
            }
            else if(out<mn)
                mn=out;
        }
    }
    cout<<mn<<endl;

}
signed main()

{
    ios_base::sync_with_stdio(false);
    cin.tie(NULL);
    cout.tie(NULL);
    #ifndef ONLINE_JUDGE
    if(fopen("INPUT.txt","r"))
    {
        freopen ("INPUT.txt" , "r" , stdin);
        freopen ("OUTPUT.txt" , "w" , stdout);
    }
    #endif
    auto clk=clock();
    mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());
    // -------------------------------------Code starts here---------------------------------------------------------------------

    int t=1;
    // cin>>t;
    for(int test=1;test<=t;test++)
    {
        // cout<<"Case #"<<test<<": ";
        // cout<<endl;
        solve();
        // cout<<endl;

    }

    // -------------------------------------Code ends here------------------------------------------------------------------


    clk = clock() - clk;
    cerr << fixed << setprecision(6) << "Time: " << ((double)clk)/CLOCKS_PER_SEC << "\n";
    return 0;
}


Measured memory_bytes: 52116