#include <bits/stdc++.h>
using namespace std;
const int k = 1000022;
char m[k];
char u[k];
char t[k];
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  ;
  int n;
  string s;
  cin >> s;
  n = s.size();
  m[n - 1] = s[n - 1];
  for (int i = n - 2; i >= 0; i--) {
    m[i] = min(s[i], m[i + 1]);
  }
  int i = 0, top = 0, tt = 0;
  while (i < n) {
    while (top && t[top] <= m[i]) u[tt++] = t[top--];
    t[++top] = s[i++];
  }
  while (top) u[tt++] = t[top--];
  for (int i = 0; i < n; i++) cout << u[i];
  cout << endl;
  return 0;
}
