diff --git a/library/description.md b/library/description.md
index 6fb04b2..adb683d 100644
--- a/library/description.md
+++ b/library/description.md
@@ -1,7 +1,8 @@
 Your library teacher 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
-Output: the number of ways you can place red-colored books and blue-colored books onto the bookshelf
+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