#include <bits/stdc++.h>
using namespace std;
int p;
string s, u = "";
multiset<char> ms;
vector<char> t;
int main() {
  cin >> s;
  for (int l = 0; l < s.size(); ++l) ms.insert(s[l]);
  for (int l = 0; l < s.size(); ++l) {
    while (t.size() > 0 && t[t.size() - 1] <= *ms.begin()) {
      u += t[t.size() - 1];
      t.pop_back();
    }
    if (s[l] == *ms.begin()) {
      ms.erase(ms.begin());
      u += s[l];
    } else {
      ms.erase(ms.lower_bound(s[l]));
      t.push_back(s[l]);
    }
  }
  for (int l = t.size() - 1; l >= 0; --l) u += t[l];
  cout << u;
  return 0;
}
