easyctf-2017/library/description.md

Your librarian has a 2-row bookshelf that can contain N books in each row. She wants to know the number of ways that she can fill the bookshelf with red-colored books and blue-colored books such that no 2 red-colored books are adjacent to each other (horizontally or vertically).

Input: the integer, N (1<=N<=2^1024)

Output: the number of ways you can place red-colored books and blue-colored books onto the bookshelf. Since this number might be really big, output it mod 10^9+7.

Ex: N=3

Input: 3

BB BB

BB BR

BR BB

RB BB

BB RB

RB BR

BR RB

Output: 7