Minesweeper Squared

During a game of Minesweeper you have arrived in the situation shown in the image, with an $n\times n$ square surrounded by $1$s, themselves surrounded by unknown cells. Your task is to determine which of the $4n+4$ surrounding $1$-cells is guaranteed to not contain a mine.

If you are unfamiliar with the rules of Minesweeper, here is a description:

  1. Each of the red-labelled cells is either empty or contains a mine. All other cells are empty.

  2. Each cell with a blue $1$ is adjacent to exactly one mine. Two cells are said to be adjacent if they share an edge or a corner. Thus, except at the border, every cell is adjacent to $8$ other cells.

Determine which of the red-labelled cells are safe, i.e., guaranteed to not contain a mine.


An integer $n$ with $1 \leq n \leq 1\, 000$, the side length of the square. The image corresponds to $n = 6$.

It can be shown that there exists at least one valid placement of mines for each $n$.


First print the number $m$ of safe cells. Then print a line with $m$ integers, the indices of the safe cells in increasing order. The cells are indexed clockwise from $1$ to $4n+4$, starting at the bottom left corner, as shown in the image.

Sample Input 1 Sample Output 1
1 3 5 7 9 11 13 15 
Sample Input 2 Sample Output 2

CPU Time limit 1 second
Memory limit 1024 MB
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Languages English, Svenska
License Creative Commons License (cc by-sa)

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