#include <bits/stdc++.h>
using namespace std;
int n,m,zc,len,i,k,cnt[55],p[1000100],st[1000100],e[1000100],last[1000100][55];
vector<int> c[55];
char s[1000100];
int id(char c) {
  if (c>='a' && c<='z') return c-'a';
  return 26+c-'A';
}
char back(int i) {
  if (i<26) return 'a'+i;
  return 'A'+i-26;
}
void mov(int i) {
  putchar(back(i));
  zc=0;
  for (int j=0; j<c[i].size(); j++) {
    int cur=c[i][j];
    if (p[cur]==last[cur][i]) cnt[i]--;
    p[cur]++;
    if (p[cur]==st[cur+1]) m++; else e[zc++]=cur;
  }
  c[i].clear();
  for (int j=0; j<zc; j++) c[id(s[p[e[j]]])].push_back(e[j]);
}
int main() {
  for (n=0, len=1; scanf("%s",s+len)!=EOF; n++) {
    for (p[n]=st[n]=len; s[len]; len++) last[n][id(s[len])]=len;
    for (i=0; i<52; i++) if (last[n][i]>0) cnt[i]++;
  }
  st[n]=len;
  for (i=0; i<52; i++) c[i].reserve(cnt[i]);
  for (i=0; i<n; i++) c[id(s[p[i]])].push_back(i);
  while (m<n) {
    for (i=0; i<52; i++) if (c[i].size()>0 && cnt[i]==c[i].size()) break;
    if (i<52) { mov(i); continue; }
    for (k=i=0; i<52; i++) if (c[i].size()>c[k].size() || (c[i].size()==c[k].size() && cnt[i]<cnt[k])) k=i;
    mov(k);
  }
  return 0;
}