A14014 | Strange Shuffle

This is an interactive problem.

$n$ people sitting in a circle are trying to shuffle a deck of cards. The players are numbered from $1$ to $n$ , so that players $i$ and $i+1$ are neighbours (as well as players $1$ and $n$ ). Each of them has exactly $k$ cards, where $k$ is even. The left neighbour of a player $i$ is player $i - 1$ , and their right neighbour is player $i + 1$ (except for players $1$ and $n$ , who are respective neighbours of each other).

Each turn the following happens: if a player has $x$ cards, they give $\lfloor x / 2 \rfloor$ to their neighbour on the left and $\lceil x / 2 \rceil$ cards to their neighbour on the right. This happens for all players simultaneously.

However, one player $p$ is the impostor and they just give all their cards to their neighbour on the right. You know the number of players $n$ and the number of cards $k$ each player has initially, but $p$ is unknown to you. Your task is to determine the value of $p$ , by asking questions like "how many cards does player $q$ have?" for an index $q$ of your choice. After each question all players will make exactly one move and give their cards to their neighbours. You need to find the impostor by asking no more than $1000$ questions.

The first line contains two integers $n$ and $k$ ( $4 \le n \le 10^5$ , $2 \le k \le 10^9$ , $k$ is even) — the number of players and the number of cards.

You can ask questions by printing "? $q$ ". The answer to this question is the number of cards player $q$ has now ( $1 \le q \le n$ ). The shuffling process starts immediately after your first question, so the answer to the first one is always equal to $k$ .

Once you have identified the impostor, you can output the answer by printing "! $p$ ", where $p$ is the player who is the impostor ( $1 \le p \le n$ ). Then you have to terminate your program.

You have to find the impostor by asking no more than $1000$ questions.

After printing a query do not forget to output end of line and flush the output. Otherwise, you will get Idleness limit exceeded. To do this, use:

- fflush(stdout) or cout.flush() in C++;
- System.out.flush() in Java;
- flush(output) in Pascal;
- stdout.flush() in Python;
- see documentation for other languages.

Hacks

To make a hack, use the following test format.

The only line of input should contain three integers $n$ , $k$ and $p$ ( $4 \le n \le 10^5$ , $2 \le k \le 10^9$ , $k$ is even, $1 \le p \le n$ ) — the number of people, the number of cards each person has initially, and the position of the impostor.