#include <bits/stdc++.h>
using namespace std;
const double eps = 1e-4;
const double PI = acos(-1.0);
const int INF = 1e9 + 10;
const int MOD = 1e9 + 7;
const int N = 1e5 + 10;
char s[N], b[N];
int main() {
  while (~scanf("%s", s)) {
    int len = strlen(s);
    b[len - 1] = s[len - 1];
    for (int i = len - 2; i >= 0; i--) b[i] = min(s[i], b[i + 1]);
    b[len] = '\0';
    stack<char> q;
    while (!q.empty()) q.pop();
    for (int i = 0; i < len; i++) {
      q.push(s[i]);
      while (!q.empty() && q.top() <= b[i + 1]) {
        printf("%c", q.top());
        q.pop();
      }
    }
    while (!q.empty()) {
      printf("%c", q.top());
      q.pop();
    }
    puts("");
  }
  return 0;
}
