#include <bits/stdc++.h>
#pragma optimize("Ofast")
using namespace std;
using ll = long long;
const int N = 1e5 + 5;
int main() {
  ios_base::sync_with_stdio(0);
  string s;
  cin >> s;
  map<char, int> cnt;
  for (int i = 0; i < s.size(); ++i) {
    cnt[s[i]]++;
  }
  stack<char> stk;
  for (int i = 0; i < s.size(); ++i) {
    char ch = s[i];
    stk.push(ch);
    cnt[ch]--;
    while (stk.size()) {
      ch = stk.top();
      bool smaller = false;
      for (char itr = 'a'; itr < ch; ++itr) {
        if (cnt[itr]) {
          smaller = true;
        }
      }
      if (smaller == false) {
        cout << stk.top();
        stk.pop();
      } else {
        break;
      }
    }
  }
  while (stk.size()) {
    cout << stk.top();
    stk.pop();
  }
  return 0;
}
