A10913. Minimal string

Petya recieved a gift of a string $s$ with length up to $10^{5}$ characters for his birthday. He took two more empty strings $t$ and $u$ and decided to play a game. This game has two possible moves:

- Extract the first character of $s$ and append $t$ with this character.
- Extract the last character of $t$ and append $u$ with this character.

Petya wants to get strings $s$ and $t$ empty and string $u$ lexicographically minimal.

You should write a program that will help Petya win the game.

First line contains non-empty string $s$ ( $1<=|s|<=10^{5}$ ), consisting of lowercase English letters.

Print resulting string $u$ .