The only difference between the easy and the hard version is the limit to the number of queries.

This is an interactive problem.

There is an array $a$ of $n$ different numbers. In one query you can ask the position of the second maximum element in a subsegment $a[l..r]$. Find the position of the maximum element in the array in no more than 20 queries.

A subsegment $a[l..r]$ is all the elements $a_l, a_{l + 1}, ..., a_r$. After asking this subsegment you will be given the position of the second maximum from this subsegment in the whole array.

The first line contains a single integer $n$ $(2 \leq n \leq 10^5)$ — the number of elements in the array.

Interaction

You can ask queries by printing "? $l$ $r$" $(1 \leq l < r \leq n)$. The answer is the index of the second maximum of all elements $a_l, a_{l + 1}, ..., a_r$. Array $a$ is fixed beforehand and can't be changed in time of interaction.

You can output the answer by printing "! $p$", where $p$ is the index of the maximum element in the array.

You can ask no more than 20 queries. Printing the answer doesn't count as a query.

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.

In the first line output a single integer $n$ $(2 \leq n \leq 10^5)$. In the second line output a permutation of $n$ integers $1$ to $n$. The position of $n$ in the permutation is the position of the maximum