#include <bits/stdc++.h>
using namespace std;
int freq[26];
char nxt = 'a';
string ans, temp;
int main() {
  string s;
  cin >> s;
  for (int i = 0; i < s.size(); i++) freq[s[i] - 'a']++;
  reverse(s.begin(), s.end());
  int k = s.size();
  for (int i = 0; i < k; i++) {
    char a = s.back();
    s.pop_back();
    temp += a;
    freq[a - 'a']--;
    while (!freq[nxt - 'a']) nxt++;
    if (nxt == 'z' + 1) continue;
    while (!temp.empty() && temp.back() <= nxt) {
      ans += temp.back();
      temp.pop_back();
    }
  }
  while (temp.size()) {
    ans += temp.back();
    temp.pop_back();
  }
  cout << ans << endl;
}
