You are given a triangle ABC and two randomly chosen points P and Q inside the triangle. How to calculate expected value of Area of the APQ ? This problem : http://acm.timus.ru/problem.aspx?space=1&num=1973.

As we known, there are N! permutations of {1,2,...N}. Now you are requested to find how many permutations satisfying that, the difference of every two adjacent numbers is not more than K. Because the answer may be quite large, you only need to output the answer module (%) 1,000,000,007. You can assume 2 <= N <= 50 and 0 <= K <= 4.

Кто может помочь решить? http://mirror.codeforces.com/group/Char5qdoff/contest/101564/attachments/download/6066/20102011-acmicpc-southwestern-european-regional-programming-contest-swerc-2010-en.pdf.