inline bool ck ()
{
	int ans (0);
	for (int i = 1; i <= top; i ++)
	{
		int t1 = cross (*stk[i + 1] - *stk[i], *stk[i + 2] - *stk[i + 1]);
		int t2 = cross (*stk[i + 2] - *stk[i + 1], *stk[i + 3] - *stk[i + 2]);
		if (t1 != 0 && t2 != 0)
			return false;
		ans += t1 + t2;
	}	
	if (ans == 0)
		return false;
	return true;
}