#include <bits/stdc++.h>
#include <atcoder/all>
using namespace std;
using namespace atcoder;
using ll = long long;
using ld = long double;

string S;
int cnt[26];

int main() {
  cin >> S;
  for (int i = 0; i < S.size(); i++) {
    cnt[S[i] - 'a']++;
  }
  cout << string(cnt[0] + 1, 'a') << endl;
  return 0;
}