561 B
561 B
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<=2^128)
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: 6