#include <cstdio>
#include <cstring>
#include <cmath>
#include <stdlib.h>
#include <queue>
#include <set>
#include <iostream>
#include <stack>
#include <map>
#include <unordered_map>
#include <vector>
#include <algorithm>
//#include <assert.h>
//#include <sstream>
using namespace std;

#define ll long long
#define ull unsigned long long
#define pii pair <int, int>
#define piii pair <int, pii>
#define vi vector <int>
#define vl vector <ll>
#define pll pair <ll, ll> 
#define PI acos(-1)
#define MP make_pair
#define PB push_back
#define F first
#define S second
#define mem(a, b) memset(a, b, sizeof(a))
#define gcd(a,b) __gcd(a,b)
#define lcm(a,b) (a*(b/gcd(a,b)))
#define sqr(a) ((a)*(a))
#define inf 100000000
const int INF = 0x3f3f3f3f;
const long long LINF = 0x3f3f3f3f3f3f3f3fLL;
#define mod 1000000007
#define mod1 1000000007
#define mod2 1000000009
#define b1 43
#define b2 41
#define EPS 1e-9
//define harmonic(n) 0.57721566490153286l+log(n)
#define nl puts("")
#define odd(n) ((n)&1)
#define even(n) (!((n)&1))
#define vsort(v) sort(v.begin(), v.end())
#define lc (node<<1)
#define rc ((node<<1)|1)
//loop
#define rep(i, n) for(int i = 0; i < n; ++i)
#define REP(i, n) for(int i = 1; i <= n; ++i)
#define forn(i, j, n) for(int i = j; i <= n; ++i)
//input
#define si(a) scanf("%d", &a)
#define sii(a, b) scanf("%d%d", &a, &b)
#define siii(a, b, c) scanf("%d%d%d", &a, &b, &c)
#define sl(a) scanf("%lld", &a)
#define sll(a, b) scanf("%lld%lld", &a, &b)
#define slll(a, b, c) scanf("%lld%lld%lld", &a, &b, &c)
#define sd(a) scanf("%lf", &a)
#define sc(a) scanf("%c", &a)
#define sst(a) scanf("%s", a)
inline bool EQ(double a, double b) { return fabs(a-b) < 1e-9; }
//debug
#ifdef tahsin

template < typename F, typename S >
ostream& operator << ( ostream& os, const pair< F, S > & p ) {
	return os << "(" << p.first << ", " << p.second << ")";
}

template < typename T >
ostream &operator << ( ostream & os, const vector< T > &v ) {
	os << "{";
	for(auto it = v.begin(); it != v.end(); ++it) {
		if( it != v.begin() ) os << ", ";
		os << *it;
	}
    return os << "}";
}

template < typename T >
ostream &operator << ( ostream & os, const set< T > &v ) {
	os << "[";
	for(auto it = v.begin(); it != v.end(); ++it) {
		if( it != v.begin()) os << ", ";
		os << *it;
	}
    return os << "]";
}

template < typename F, typename S >
ostream &operator << ( ostream & os, const map< F, S > &v ) {
	os << "[";
	for(auto it = v.begin(); it != v.end(); ++it) {
		if( it != v.begin() ) os << ", ";
		os << it -> first << " = " << it -> second ;
	}
    return os << "]";
}

#define dbg(args...) do {cerr << #args << " : "; faltu(args); } while(0)

clock_t tStart = clock();
#define timeStamp dbg("Execution Time: ", (double)(clock() - tStart)/CLOCKS_PER_SEC)

void faltu () { cerr << endl; }

template <typename T>
void faltu( T a[], int n ) {
	for(int i = 0; i < n; ++i) cerr << a[i] << ' ';
	cerr << endl;
}

template <typename T, typename ... hello>
void faltu( T arg, const hello &... rest) { cerr << arg << ' '; faltu(rest...); }

#else
#define dbg(args...)
#endif

ll add(ll a, ll b, ll m = mod) {
	ll ret = a+b;
	if(ret >= m) ret -= m;
	return ret;
}

ll subtract(ll a, ll b, ll m = mod) {
	ll ret = a-b;
	if(ret < 0) ret += m;
	return ret;
}

ll mult(ll a, ll b, ll m = mod) {
	ll ret = a*b;
	if(ret >= m) ret %= m;
	return ret;
}

ll bigmod(ll a, ll b, ll m = mod) {
	ll ret = 1;
	while(b) {
		if(b&1) ret = mult(ret, a, m);
		b >>= 1; a = mult(a, a, m);
	}
	return ret;
}

ll inverse(ll n, ll m = mod) { return bigmod(n, mod-2, m); }

/*
bool base[1000010];
vi primes;
void sieve(int mx) {
	mx += 10;
	int x = sqrt(mx);
	for(int i = 3; i <= x; i += 2) if(base[i] == 0) for(int j = i*i, k = i<<1; j < mx; j += k) base[j] = 1;
	primes.PB(2);
	for(int i = 3; i < mx; i += 2) if(base[i] == 0) primes.PB(i);
}
*/


//Direction Array 
//int fx[]={1, -1, 0, 0}; int fy[]={0, 0, 1, -1};
//int fx[]={0, 0, 1, -1, -1, 1, -1, 1}; int fy[]={-1, 1, 0, 0, 1, 1, -1, -1};

//bit manipulation
/*
bool checkBit(int n, int i) { return (n&(1<<i)); }
int setBit(int n, int i) { return (n|(1<<i)); }
int resetBit(int n, int i) { return (n&(~(1<<i))); }
*/
//end of template


#define mxn 200010
char a[mxn];
int n, nxt[mxn][26], dp[mxn];

int f(int i) {
	if(i == n) return INF;

	int &ret = dp[i];
	if(ret != -1) return ret;

	ret = INF;

	rep(j, 26) if(nxt[i][j] == -1) return ret = 1;
	rep(j, 26) ret = min(ret, 1 + f(nxt[i][j]));
	return ret;
}

void show(int i) {
	int &ret = dp[i];

	rep(j, 26) if(nxt[i][j] == -1) {
		printf("%c\n", j+'a');
		return;
	}

	rep(j, 26) if(1 + f(nxt[i][j]) == ret) {
		printf("%c", j+'a');
		show(nxt[i][j]);
		return;
	}
}

int main () {
#ifdef tahsin
	freopen("in", "r", stdin);
//	freopen("out", "w", stdout);
#endif

	while(sst(a+1) != EOF) {
		n = strlen(a+1);
		rep(i, 26) nxt[n][i] = -1;

		for(int i = n-1; i >= 0; --i) {
			rep(j, 26) nxt[i][j] = nxt[i+1][j];
			nxt[i][a[i+1]-'a'] = i+1;
		}

		mem(dp, -1);
		f(0);
		show(0);
	}

//	timeStamp;
	return 0;
}

Measured memory_bytes: 21760