Counting Bishops

Your task is to count the number of ways k bishops can be placed on an n \times n chessboard so that no two bishops attack each other. Two bishops attack each other if they are on the same diagonal. Input The only input line has two integers n and k: the board size and the number of bishops. Output Print one integer: the number of ways modulo 10^9+7. Constraints - 1 \le n \le 500 - 1 \le k