Problem 6

通常第6题会比较难，今年的不一样？试着解，不对的话请指正。

There are n! different ways to arrange the jumps.
Given a point p between 0 and s = a_1+a_2+...a_n, it
has at most (n-1)! ways to reach. So the total ways to be matched is (n-1)(n-1)! which is less than n!. Also it
means there is at least one way that the grasshopper never lands on any point in M.